Isoptin Retard (Verapamil)
Non-Dihydropyridine Calcium Channel Blocker for High Blood Pressure and Angina
Quick Facts About Isoptin Retard
Key Takeaways About Isoptin Retard
- Dual cardiovascular action: Verapamil lowers blood pressure by dilating blood vessels and also slows heart rate by acting on the cardiac conduction system, making it effective for both hypertension and angina
- Avoid with beta-blockers: Combining verapamil with beta-blockers can cause dangerously slow heart rate, heart block, or heart failure – this combination requires extreme caution and specialist supervision
- Constipation is the most notable side effect: Up to 40% of patients experience constipation, which can often be managed with dietary fibre and adequate fluid intake
- Do not crush sustained-release tablets: The extended-release formulation must be swallowed whole to ensure proper drug release; crushing can cause a dangerous dose to be released at once
- Contraindicated with ivabradine: Verapamil must never be taken together with ivabradine due to the risk of severe bradycardia and cardiac conduction disturbances
What Is Isoptin Retard and What Is It Used For?
Isoptin Retard contains the active ingredient verapamil hydrochloride, a non-dihydropyridine calcium channel blocker that lowers blood pressure by widening blood vessels and reduces heart rate by slowing electrical conduction through the heart. It is prescribed to treat high blood pressure (hypertension) and angina pectoris (chest pain caused by reduced blood flow to the heart).
Verapamil belongs to the phenylalkylamine subclass of calcium channel blockers. It works by blocking voltage-dependent L-type calcium channels in both vascular smooth muscle and cardiac muscle cells. By preventing calcium ions from entering these cells, verapamil produces two distinct therapeutic effects: it relaxes blood vessel walls, reducing peripheral vascular resistance and lowering blood pressure, and it slows the electrical impulse through the atrioventricular (AV) node in the heart, reducing heart rate.
In patients with high blood pressure (hypertension), the vasodilatory effect of verapamil reduces the resistance that blood encounters as it flows through arteries. This decrease in peripheral vascular resistance results in lower blood pressure, which in turn reduces the workload on the heart and helps protect against long-term complications such as stroke, heart attack, kidney disease, and heart failure. The European Society of Cardiology (ESC) and the American Heart Association (AHA) recognise calcium channel blockers as a first-line treatment option for hypertension, though non-dihydropyridine agents like verapamil are generally reserved for patients who also have conditions that benefit from heart rate control, such as supraventricular tachycardia or rate control in atrial fibrillation.
In patients with angina pectoris (chest pain), verapamil works through multiple mechanisms. It increases blood flow through the coronary arteries by relaxing the coronary vasculature, thereby delivering more oxygen to the heart muscle. At the same time, it reduces the heart's demand for oxygen by slowing the heart rate and decreasing the force of cardiac contraction. This dual mechanism makes verapamil particularly effective for chronic stable angina and vasospastic (Prinzmetal's) angina, where coronary artery spasm plays a key role.
The sustained-release (Retard) formulation of Isoptin is designed to release verapamil gradually over an extended period, providing consistent therapeutic blood levels throughout the day. This allows for once- or twice-daily dosing instead of the three-times-daily dosing required with immediate-release verapamil, improving convenience and treatment adherence. The sustained-release matrix system uses sodium alginate and other excipients to control drug release.
Verapamil was one of the first calcium channel blockers to be introduced into clinical practice, having been approved for medical use in 1962. It remains widely used worldwide and is included on the World Health Organization's List of Essential Medicines. Unlike the more commonly prescribed dihydropyridine calcium channel blockers (such as amlodipine and nifedipine), verapamil has significant effects on the heart's electrical conduction system, which makes it useful for treating certain types of arrhythmias as well as hypertension and angina.
What Should You Know Before Taking Isoptin Retard?
Before starting Isoptin Retard, tell your doctor about all your medical conditions, particularly any heart problems, liver or kidney disease, and neuromuscular disorders. Verapamil has several important contraindications and must not be combined with certain other medications, including ivabradine.
Contraindications
You should not take Isoptin Retard if any of the following apply to you:
- Allergy to verapamil hydrochloride or any of the other ingredients in the tablet – symptoms may include rash, itching, swelling, or difficulty breathing
- Certain serious heart conditions such as heart failure or significant left ventricular dysfunction – verapamil's negative inotropic effect (reducing the force of heart contraction) can worsen heart failure
- Heart block (second or third degree) – disorders of the cardiac conduction system, unless you have a functioning pacemaker
- Sick sinus syndrome – unless you have a functioning pacemaker
- Acute myocardial infarction complicated by slow heart rate (bradycardia), markedly low blood pressure (hypotension), or impaired left ventricular function
- Atrial fibrillation or flutter with Wolff-Parkinson-White (WPW) syndrome – verapamil can accelerate conduction through the accessory pathway, potentially causing ventricular fibrillation
- Digitalis toxicity – verapamil may worsen the cardiac effects of digitalis overdose
- Concurrent use of ivabradine – the combination can cause severe bradycardia and other dangerous cardiac conduction disturbances
Warnings and Precautions
Talk to your doctor or pharmacist before taking Isoptin Retard if you have or have had any of the following conditions:
- Low blood pressure (hypotension) – verapamil can further lower blood pressure, potentially causing dizziness or fainting
- Slow heart rate (bradycardia) – verapamil slows the heart rate and may cause clinically significant bradycardia
- Aortic stenosis (narrowing of the aortic valve) – the blood vessel-dilating effects can compromise cardiac output
- Heart failure or impaired left ventricular function – verapamil reduces the force of cardiac contraction and should only be used under specialist supervision
- First-degree heart block – verapamil may prolong the PR interval and worsen conduction disturbances
- Liver disease – verapamil is extensively metabolised by the liver, and impaired hepatic function can lead to significantly higher drug levels, potentially requiring dose adjustment. In severe liver disease, the elimination half-life may increase to 14–16 hours
- Kidney disease – although verapamil is primarily metabolised by the liver, renal impairment may affect the elimination of metabolites
- Neuromuscular diseases such as myasthenia gravis, Lambert-Eaton syndrome, or Duchenne muscular dystrophy – verapamil may reduce neuromuscular transmission and worsen muscle weakness
Intravenous beta-blockers should never be administered to patients who are taking verapamil, as this combination can cause severe bradycardia, complete heart block, or asystole (cardiac arrest). If you are taking oral beta-blockers, your doctor must carefully monitor your heart rhythm and blood pressure. Always tell any healthcare provider that you are taking verapamil before receiving any treatment, including anaesthesia for surgery.
Pregnancy and Breastfeeding
If you are pregnant, think you might be pregnant, or are planning to become pregnant, inform your doctor before taking Isoptin Retard. Verapamil crosses the placenta and there is a potential risk of harm to the developing foetus. Animal studies have suggested possible adverse effects on foetal development, and although controlled human studies are limited, verapamil should only be used during pregnancy if the expected benefit to the mother clearly outweighs the potential risk to the foetus.
Verapamil is excreted in breast milk at concentrations that may have pharmacological effects on the nursing infant. Therefore, breastfeeding is not recommended during treatment with Isoptin Retard. If breastfeeding is desired, your doctor may advise switching to an alternative medication with a better-established safety profile for nursing mothers.
Driving and Operating Machinery
Isoptin Retard can cause dizziness, lightheadedness, or fatigue in some patients, particularly at the start of treatment or after a dose increase. These effects can impair your ability to drive a car or operate machinery safely. You should assess your own ability to perform these activities before engaging in them. If you experience any symptoms that affect your concentration or reaction time, avoid driving and operating machinery until the symptoms resolve. Consult your doctor if these effects persist.
Food and Drink Interactions
Grapefruit juice can increase the blood levels of verapamil by inhibiting the CYP3A4 enzyme responsible for its metabolism. This can lead to an unpredictable enhancement of the drug's effects, potentially causing excessive blood pressure lowering, dizziness, or cardiac conduction disturbances. It is recommended to avoid grapefruit and grapefruit juice while taking Isoptin Retard.
Alcohol: In some patients, verapamil may alter the way alcohol is metabolised, potentially increasing its effects. Exercise caution when consuming alcohol during treatment. Your doctor can advise you on safe limits.
Sodium Content
Isoptin Retard 120 mg contains less than 1 mmol (23 mg) sodium per tablet, meaning it is essentially “sodium-free”. The 180 mg tablet contains 27.9 mg sodium per tablet (1.4% of the recommended maximum daily intake), and the 240 mg tablet contains 37.1 mg sodium per tablet (1.9% of the recommended maximum daily intake). This may be relevant for patients on a sodium-restricted diet or those monitoring their sodium intake for medical reasons.
How Does Isoptin Retard Interact with Other Drugs?
Verapamil has a large number of clinically important drug interactions due to its effects on heart rate, cardiac conduction, and hepatic metabolism. It is metabolised by CYP3A4 and itself inhibits CYP3A4 and P-glycoprotein, which can increase blood levels of many other drugs. Always inform your doctor about all medications you are taking.
Drug interactions with verapamil can occur through several mechanisms. As a substrate and inhibitor of CYP3A4, verapamil can both be affected by and affect the metabolism of numerous other drugs. It also inhibits P-glycoprotein (an important drug transporter), which can increase the absorption and reduce the elimination of drugs such as digoxin and dabigatran. Additionally, the pharmacodynamic effects of verapamil on heart rate, cardiac conduction, and blood pressure can interact additively or synergistically with other cardiovascular medications.
Major Interactions
| Drug | Category | Effect | Recommendation |
|---|---|---|---|
| Ivabradine | Heart rate-lowering agent | Severe additive bradycardia and cardiac conduction disturbances | Combination is contraindicated – must not be used together |
| Beta-blockers (e.g. metoprolol, atenolol, propranolol) | Antihypertensive / Anti-anginal | Additive negative effects on heart rate, AV conduction, and cardiac contractility; risk of heart block, severe bradycardia, or heart failure | IV beta-blockers contraindicated; oral combination only under close specialist supervision with ECG monitoring |
| Digoxin | Cardiac glycoside | Verapamil inhibits P-glycoprotein, increasing digoxin blood levels by 50–75%; additive AV nodal depression | Reduce digoxin dose by 50%; monitor digoxin levels and heart rate closely |
| Dabigatran | Direct oral anticoagulant (DOAC) | Verapamil inhibits P-glycoprotein, increasing dabigatran blood levels and bleeding risk | Reduce dabigatran dose; take verapamil and dabigatran at the same time (not 2 hours apart) |
| Quinidine / Flecainide | Antiarrhythmic agents | Additive cardiac depression; risk of severe hypotension and conduction disturbances | Avoid combination or use only under close cardiac monitoring |
| Doxorubicin | Chemotherapy agent | Verapamil may increase doxorubicin blood levels and enhance its cardiotoxicity | Avoid combination; discuss with oncologist |
Moderate Interactions
| Drug | Category | Effect | Recommendation |
|---|---|---|---|
| Simvastatin / Lovastatin / Atorvastatin | Cholesterol-lowering (statins) | CYP3A4 inhibition by verapamil increases statin levels, raising the risk of myopathy and rhabdomyolysis | Limit simvastatin to 20 mg/day and lovastatin to 40 mg/day; use lowest effective statin dose |
| Cyclosporine / Tacrolimus / Sirolimus | Immunosuppressants | Verapamil increases blood levels of these immunosuppressants via CYP3A4 and P-glycoprotein inhibition | Monitor immunosuppressant levels closely; dose reduction usually required |
| Theophylline | Bronchodilator (asthma) | Verapamil reduces theophylline clearance, increasing blood levels and risk of toxicity | Monitor theophylline levels; dose adjustment may be needed |
| Carbamazepine | Antiepileptic | Verapamil increases carbamazepine levels; carbamazepine may reduce verapamil levels | Monitor carbamazepine levels; adjust doses as needed |
| Metformin | Antidiabetic | Verapamil may reduce the glucose-lowering effect of metformin | Monitor blood glucose more frequently; dose adjustment of metformin may be needed |
| Rifampicin | Antibiotic (TB treatment) | Strong CYP3A4 inducer that markedly reduces verapamil blood levels, potentially abolishing its therapeutic effect | Avoid combination if possible; if unavoidable, monitor blood pressure closely and consider dose increase |
| Clarithromycin / Erythromycin / Telithromycin | Macrolide antibiotics | CYP3A4 inhibition increases verapamil levels | Monitor blood pressure and heart rate; temporary verapamil dose reduction may be needed |
| Lithium | Mood stabiliser | Unpredictable changes in lithium levels; possible increased neurotoxicity | Monitor lithium levels closely when starting or stopping verapamil |
| Colchicine | Gout treatment | Verapamil inhibits P-glycoprotein, increasing colchicine levels and risk of toxicity | Reduce colchicine dose; avoid in patients with renal or hepatic impairment |
| St. John’s Wort (Hypericum perforatum) | Herbal supplement | CYP3A4 inducer that may reduce verapamil effectiveness | Avoid concurrent use or monitor blood pressure closely |
Verapamil can also interact with anaesthetic agents used during surgery, including inhaled anaesthetics and neuromuscular blocking agents, potentially enhancing their cardiovascular depressant effects. If you are scheduled for surgery, it is essential to inform your anaesthesiologist that you are taking verapamil. In general, verapamil treatment is continued up to and through surgery, but the anaesthesiologist will adjust their approach accordingly.
What Is the Correct Dosage of Isoptin Retard?
The recommended dose of Isoptin Retard for adults is 120–240 mg taken once or twice daily. Your doctor will determine the appropriate dose based on your medical condition, response to treatment, and tolerance. Always take the sustained-release tablets whole – do not crush, chew, or break them (except the 240 mg tablet, which may be split in half).
Always take Isoptin Retard exactly as your doctor has prescribed. Do not change your dose or stop taking the medication without first consulting your doctor. The sustained-release tablets are designed to release the drug gradually and should be swallowed whole with a glass of water, preferably with or after food.
Adults
Hypertension (High Blood Pressure)
Usual dose: 120–240 mg once or twice daily
Maximum dose: 480 mg daily (in divided doses)
Your doctor will typically start with a lower dose and increase it gradually based on your blood pressure response. The full antihypertensive effect may take 1–2 weeks to develop. Dosing is individualised, and the goal is to achieve effective blood pressure control with the lowest dose that produces minimal side effects.
Angina Pectoris (Chest Pain)
Usual dose: 120–240 mg once or twice daily
Maximum dose: 480 mg daily (in divided doses)
The dose is adjusted based on the frequency and severity of angina episodes. Verapamil reduces the heart's oxygen demand and increases coronary blood flow, preventing angina attacks. It does not provide immediate relief during an acute episode – short-acting nitrates should be used for that purpose.
Tablet Splitting Information
180 mg tablet: The score line on the 180 mg tablet is not for dose splitting. It is present only to make the tablet easier to swallow. Do not attempt to split this tablet into equal halves for dosing purposes.
240 mg tablet: This tablet can be split along the score line into two equal 120 mg doses if directed by your doctor.
Children and Adolescents
There are no established data on the use of Isoptin Retard sustained-release tablets in children and adolescents. Immediate-release verapamil may be used in paediatric patients for specific cardiac indications (such as supraventricular tachycardia), but this requires specialist supervision and dosing based on body weight. The sustained-release formulation is not intended for paediatric use.
Elderly Patients
Elderly patients may be more sensitive to the effects of verapamil due to age-related changes in liver function and cardiac conduction. Your doctor may start with a lower dose and increase it more cautiously. Blood pressure, heart rate, and ECG should be monitored regularly during treatment, particularly at the start and after dose changes.
Patients with Liver or Kidney Impairment
Verapamil is extensively metabolised by the liver. In patients with hepatic impairment, the bioavailability of verapamil can increase significantly (up to 4-fold in severe liver disease), and the elimination half-life may be prolonged to 14–16 hours. Your doctor will prescribe a substantially lower dose and monitor you carefully. In patients with renal impairment, no specific dose adjustment is routinely required, but active metabolites may accumulate. Your doctor will consider renal function when determining the appropriate dose.
Missed Dose
If you forget to take a dose, skip it and take your next dose at the usual time. Do not take a double dose to make up for the missed one. If you frequently forget to take your medication, consider setting a daily alarm or taking the tablet alongside a routine activity such as a meal.
Overdose
An overdose of verapamil can be life-threatening. Symptoms may include severe hypotension (dangerously low blood pressure), bradycardia (very slow heart rate), heart block, cardiac arrest, and loss of consciousness. In severe cases, cardiogenic shock and metabolic acidosis can develop. The sustained-release formulation may cause delayed symptom onset, as the drug continues to be absorbed for many hours after ingestion. Seek immediate emergency medical attention if you suspect an overdose – contact your local emergency services or poison control centre immediately. Do not wait for symptoms to appear.
Stopping Isoptin Retard
Do not stop taking Isoptin Retard without first consulting your doctor. Your doctor will advise you on how long you need to take this medication. In some cases, particularly if you are taking verapamil for angina, sudden discontinuation may cause a worsening of symptoms (rebound angina). Your doctor may recommend tapering the dose gradually rather than stopping abruptly.
What Are the Side Effects of Isoptin Retard?
The most common side effects of Isoptin Retard include constipation, nausea, slow heart rate (bradycardia), low blood pressure, headache, dizziness, fatigue, ankle swelling, and flushing. Constipation is particularly common and can often be managed with dietary changes. Most side effects are mild, but some require immediate medical attention.
Like all medicines, Isoptin Retard can cause side effects, although not everyone gets them. Most side effects are dose-dependent and tend to be more common at higher doses. If any side effects become troublesome or persistent, consult your doctor or pharmacist.
- Swelling of the face, tongue, or throat (angioedema) – very rare but potentially life-threatening
- Difficulty swallowing or breathing
- Hives (urticaria) combined with breathing difficulties
- Severe dizziness, fainting, or loss of consciousness (may indicate dangerously low blood pressure or heart block)
- Severe chest pain or irregular heartbeat
Common
May affect up to 1 in 10 people
- Constipation
- Nausea
- Slow heart rate (bradycardia)
- Heart failure or worsening of existing heart failure
- Low blood pressure (hypotension)
- Headache
- Dizziness
- Fatigue and drowsiness
- Ankle and foot swelling (peripheral oedema)
- Itching and hives (urticaria)
- Flushing (warmth and redness of the face)
Uncommon
May affect up to 1 in 100 people
- Chest pain (angina-like symptoms)
- Rapid heart rate (tachycardia), palpitations
- Changes in heart rhythm
- Elevated liver enzymes
- Erectile dysfunction
- Loss of appetite
- Skin rash
- Tingling or numbness (paraesthesia)
- Diarrhoea, abdominal pain
- Muscle cramps, tremor
- Confusion, sleep disturbances
- Frequent urination
Rare
May affect up to 1 in 1,000 people
- Hypersensitivity reactions
- Fainting (syncope)
- Hormonal changes (elevated prolactin levels)
- Galactorrhoea (milk discharge from the breast)
- Vomiting
- Blurred vision
- Tinnitus (ringing in the ears)
- Excessive sweating
Very Rare
May affect fewer than 1 in 10,000 people
- Angioedema (swelling of the face, tongue, or throat)
- Vertigo
- Gynaecomastia (breast enlargement in men)
- Gingival hyperplasia (swelling of the gums)
- Ileus (intestinal obstruction)
- Photosensitivity (skin reaction to sunlight)
- Stevens-Johnson syndrome (severe skin and mucous membrane reaction)
- Erythema multiforme
- Vasculitis with purpura (inflammation of blood vessels with skin bleeding)
- Joint pain (arthralgia), muscle pain (myalgia)
Frequency not known: Hair loss (alopecia), extrapyramidal symptoms (movement disorders including tremor, involuntary movements, muscle stiffness, restlessness), serious cardiac rhythm disturbances, hyperkalaemia (high potassium levels), and renal failure have been reported but their frequency cannot be determined from available data.
Constipation is the most commonly reported side effect of verapamil. It is usually most pronounced at the start of treatment and may improve over time. To help prevent or manage constipation, increase your intake of dietary fibre (fruits, vegetables, whole grains), drink plenty of water, and stay physically active. If constipation persists, your doctor may recommend a mild laxative. Do not discontinue verapamil without consulting your doctor.
How Should You Store Isoptin Retard?
Store Isoptin Retard sustained-release tablets at or below 25°C in the original packaging, out of the reach and sight of children. Do not use after the expiry date printed on the packaging.
Store the tablets in the original blister packaging to protect them from moisture and light. Keep the tablets at a temperature not exceeding 25°C (77°F). Do not refrigerate or freeze. Check the expiry date (marked “EXP” on the carton and blister) before taking any tablets – the expiry date refers to the last day of the stated month.
Keep this medicine out of the sight and reach of children. Do not flush unused tablets down the toilet or throw them in household waste. Return any unused or expired medication to your pharmacy for safe disposal, helping to protect the environment from pharmaceutical contamination.
What Does Isoptin Retard Contain?
Each Isoptin Retard sustained-release tablet contains verapamil hydrochloride as the active ingredient, available in 120 mg, 180 mg, and 240 mg strengths. The tablets contain various inactive ingredients necessary for the sustained-release mechanism and tablet manufacture.
Active Ingredient
The active substance is verapamil hydrochloride. Each sustained-release tablet contains 120 mg, 180 mg, or 240 mg of verapamil hydrochloride. The immediate-release film-coated tablets (Isoptin, without “Retard”) contain 40 mg of verapamil hydrochloride per tablet.
Inactive Ingredients (Excipients)
The other ingredients are: sodium alginate, microcrystalline cellulose, povidone, magnesium stearate, purified water, talc, hypromellose, macrogol 400, macrogol 6000, and montan glycol wax. These are standard pharmaceutical excipients that control the sustained release of the drug and ensure proper tablet formation.
Colouring Agents
- 120 mg tablets: Titanium dioxide (E171)
- 180 mg tablets: Titanium dioxide (E171), iron oxide (E172)
- 240 mg tablets: Titanium dioxide (E171), quinoline yellow (E104), indigo carmine (E132)
Tablet Appearance and Packaging
120 mg: White, round, convex sustained-release tablet, 10 mm in diameter, imprinted “KNOLL, 120 SR”.
180 mg: Pink, oval sustained-release tablet with a score line, imprinted “KNOLL/SR 180”.
240 mg: Yellow-green, oblong, convex sustained-release tablet with a score line.
Available in calendar packs of 28 and 98 tablets. Not all pack sizes may be marketed in your country.
How Does Verapamil Work in the Body?
Verapamil blocks L-type calcium channels in both cardiac muscle and vascular smooth muscle, producing a dual therapeutic effect. In the heart, it slows conduction through the AV node and reduces contractility. In blood vessels, it causes vasodilation and lowers peripheral vascular resistance. This combination of cardiac and vascular effects distinguishes verapamil from dihydropyridine calcium channel blockers.
Calcium ions play a critical role in the contraction of muscle cells. When calcium enters cardiac and smooth muscle cells through voltage-dependent L-type calcium channels, it triggers a cascade of intracellular events leading to muscle contraction. Verapamil selectively blocks these L-type calcium channels, preventing calcium influx and thereby inhibiting contraction in both vascular smooth muscle and cardiac muscle.
Vascular Effects
By blocking calcium channels in vascular smooth muscle, verapamil causes the blood vessel walls to relax and dilate. This vasodilation reduces peripheral vascular resistance (the resistance that the heart must pump against), which in turn lowers systemic blood pressure. This is the primary mechanism by which verapamil treats hypertension. In the coronary arteries specifically, vasodilation increases blood flow to the heart muscle, which is beneficial for patients with angina, particularly vasospastic (Prinzmetal's) angina where coronary artery spasm is the underlying cause.
Cardiac Effects
Unlike the dihydropyridine calcium channel blockers (such as amlodipine and nifedipine), verapamil has pronounced effects on cardiac tissue. It slows electrical conduction through the sinoatrial (SA) node and atrioventricular (AV) node, reducing heart rate (negative chronotropic effect). It also reduces the force of cardiac contraction (negative inotropic effect) and slows AV conduction (negative dromotropic effect). These cardiac effects are therapeutically useful for controlling heart rate in conditions such as supraventricular tachycardia and atrial fibrillation, and for reducing myocardial oxygen demand in angina.
Pharmacokinetic Profile
After oral administration, verapamil is well absorbed from the gastrointestinal tract (more than 90% of the dose). However, it undergoes extensive first-pass metabolism in the liver, resulting in an oral bioavailability of only 20–35%. The principal metabolic pathway involves the CYP3A4, CYP1A2, CYP2C8, CYP2C9, and CYP2C18 enzymes. The primary active metabolite is norverapamil, which retains approximately 20% of the cardiovascular activity of the parent drug.
The elimination half-life of immediate-release verapamil is approximately 6–8 hours after a single dose, increasing to 4.5–12 hours with repeated dosing due to saturable first-pass metabolism. The sustained-release (Retard) formulation extends effective drug release, allowing once- or twice-daily dosing. Verapamil is highly protein-bound (approximately 90%) and is distributed widely in body tissues. Approximately 70% of the administered dose is excreted by the kidneys as metabolites, and 16% is excreted in the faeces. Less than 5% is excreted unchanged in the urine.
With repeated dosing, the bioavailability of verapamil increases because the first-pass metabolism becomes partially saturated. This means that steady-state blood levels are higher than would be predicted from single-dose pharmacokinetics, which is an important consideration when initiating or adjusting therapy.
Frequently Asked Questions About Isoptin Retard
Isoptin Retard (verapamil) is used to treat high blood pressure (hypertension) and chest pain caused by angina pectoris. It works by relaxing blood vessels to lower blood pressure and by slowing the heart rate and reducing the heart's oxygen demand. Verapamil is a non-dihydropyridine calcium channel blocker, which means it affects both the blood vessels and the heart, unlike dihydropyridine CCBs (such as amlodipine) that primarily affect blood vessels.
The most common side effects include constipation (which is particularly common with verapamil and can affect up to 40% of patients), nausea, slow heart rate, low blood pressure, headache, dizziness, fatigue, ankle swelling, itching, and facial flushing. Constipation can usually be managed by increasing dietary fibre intake, drinking more water, and staying active. Most side effects are mild and may improve as your body adjusts to the medication.
Combining verapamil with beta-blockers is potentially dangerous because both drug classes slow the heart rate and reduce cardiac conduction. This combination can lead to severe bradycardia (dangerously slow heart rate), heart block, or heart failure. Intravenous beta-blockers are strictly contraindicated with verapamil. Oral beta-blockers may sometimes be used with verapamil, but only under close specialist supervision with regular ECG monitoring. Always consult your doctor before combining these medications.
Isoptin Retard sustained-release tablets should be swallowed whole with a glass of water, preferably with or after food. Do not crush, chew, or break the tablets, as this can destroy the sustained-release mechanism and cause a dangerously large dose to be released at once. The only exception is the 240 mg tablet, which can be split along its score line into two equal halves. The score line on the 180 mg tablet is only to make swallowing easier and should not be used for dose splitting.
Grapefruit juice contains compounds (furanocoumarins) that inhibit the CYP3A4 enzyme in the gut wall and liver, which is responsible for breaking down verapamil. This inhibition can lead to significantly higher-than-expected blood levels of verapamil, potentially causing excessive blood pressure lowering, dizziness, abnormally slow heart rate, or other cardiovascular side effects. It is best to avoid grapefruit and grapefruit juice entirely while taking Isoptin Retard.
Isoptin Retard should only be used during pregnancy if the potential benefit to the mother clearly outweighs the potential risk to the foetus. Verapamil crosses the placenta, and although teratogenic effects have not been conclusively demonstrated in humans, animal studies have suggested possible harm at high doses. Verapamil is also excreted in breast milk, so breastfeeding is not recommended during treatment. If you are pregnant, planning pregnancy, or breastfeeding, discuss alternative medications with your doctor.
References
This article is based on the following international medical guidelines and peer-reviewed sources. All medical claims have evidence level 1A, the highest quality of evidence based on systematic reviews of randomised controlled trials.
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- World Health Organization (WHO). WHO Model List of Essential Medicines – 23rd list. Geneva: WHO; 2023.
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Editorial Team
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