Monoket OD
Isosorbide Mononitrate – Organic Nitrate for Angina Prevention
Quick Facts About Monoket OD
Key Takeaways About Monoket OD
- Prevention, not rescue: Monoket OD is used to prevent angina attacks – it does not work fast enough to relieve acute chest pain
- Never combine with PDE5 inhibitors: Taking Monoket OD with sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra) can cause a life-threatening drop in blood pressure
- Once-daily morning dosing: The extended-release capsule is designed to be taken once in the morning to maintain a nitrate-free interval at night and prevent tolerance
- Headache is expected initially: Headache is the most common side effect but typically improves within the first 1–2 weeks of treatment
- Do not chew or crush: Extended-release capsules must be swallowed whole to ensure proper drug release over time
What Is Monoket OD and What Is It Used For?
Monoket OD contains isosorbide mononitrate, an organic nitrate vasodilator used for the long-term prophylactic treatment of angina pectoris. It works by dilating both venous and arterial blood vessels, reducing the heart's oxygen demand and improving coronary blood flow.
Angina pectoris is a condition characterised by chest pain or discomfort that occurs when the heart muscle does not receive enough oxygen-rich blood. This most commonly happens during physical exertion, emotional stress, or exposure to cold weather. Monoket OD is prescribed specifically to prevent these angina episodes from occurring – it is not designed to treat an angina attack that is already in progress.
The active ingredient, isosorbide mononitrate, belongs to the organic nitrate class of medications. After ingestion, it is metabolised to release nitric oxide (NO), a potent signalling molecule that activates the enzyme guanylate cyclase in vascular smooth muscle cells. This leads to increased production of cyclic guanosine monophosphate (cGMP), which causes relaxation and dilation of blood vessels. The primary effect is on the venous system (capacitance vessels), which reduces the volume of blood returning to the heart (preload). This decreases ventricular wall tension and cardiac oxygen consumption, thereby preventing angina.
Isosorbide mononitrate also dilates the coronary arteries and collateral vessels, which can improve blood flow to ischaemic areas of the heart muscle. Unlike its parent compound isosorbide dinitrate, isosorbide mononitrate does not undergo first-pass hepatic metabolism, meaning it has nearly 100% oral bioavailability. This pharmacokinetic advantage results in more predictable blood levels and a more consistent clinical effect.
Monoket OD is formulated as extended-release capsules, allowing once-daily dosing. This formulation is particularly important because continuous nitrate exposure leads to tolerance – a phenomenon where the medication becomes progressively less effective. The once-daily morning dose ensures that plasma nitrate levels are low during the night, providing a crucial “nitrate-free interval” that helps maintain the drug's efficacy over long-term use.
Isosorbide mononitrate is the active metabolite of isosorbide dinitrate, another commonly used nitrate. Because it does not require liver conversion, isosorbide mononitrate offers more predictable absorption and dosing. The European Society of Cardiology (ESC) includes long-acting nitrates as a recommended option for anti-anginal therapy in patients with chronic coronary syndromes.
What Should You Know Before Taking Monoket OD?
Before starting Monoket OD, tell your doctor about all your medical conditions and medications, especially PDE5 inhibitors or riociguat. Monoket OD is contraindicated in severe hypotension, cardiogenic shock, hypertrophic obstructive cardiomyopathy, constrictive pericarditis, cardiac tamponade, and severe anaemia.
Contraindications
Monoket OD must not be used if you have any of the following conditions. Using this medication in these situations could be dangerous or even life-threatening:
- Allergy to isosorbide mononitrate, other nitrates, or any excipients – hypersensitivity reactions including angioedema have been reported
- Severe hypotension – systolic blood pressure below 90 mmHg, as further blood pressure reduction could compromise vital organ perfusion
- Cardiogenic shock – when the heart cannot pump enough blood to meet the body's needs, unless adequate left ventricular filling pressure is maintained by an intra-aortic balloon pump or similar support
- Hypertrophic obstructive cardiomyopathy (HOCM) – nitrate-induced reduction in preload can worsen the obstruction of blood flow from the left ventricle
- Constrictive pericarditis – inflammation of the pericardium that restricts normal cardiac filling
- Cardiac tamponade – accumulation of fluid in the pericardial space that compresses the heart and impairs filling
- Acute circulatory failure (shock or circulatory collapse) – conditions where further vasodilation would be harmful
- Severe hypovolaemia – critically reduced blood volume where vasodilation could cause cardiovascular collapse
- Concurrent use of riociguat – a soluble guanylate cyclase stimulator used for pulmonary hypertension; the combination can cause severe hypotension
- Severe anaemia – reduced oxygen-carrying capacity, which nitrates can further compromise
You must never take Monoket OD together with phosphodiesterase type 5 (PDE5) inhibitors such as sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra). Both nitrates and PDE5 inhibitors cause vasodilation through the cGMP pathway, and their combined use can result in a severe, potentially life-threatening drop in blood pressure that may damage the heart or brain. Do not stop taking Monoket OD in order to take a PDE5 inhibitor – consult your doctor about alternative options.
Warnings and Precautions
Talk to your doctor or pharmacist before taking Monoket OD if you have any of the following conditions, as special care or dose adjustments may be needed:
- Recent acute myocardial infarction (heart attack) – nitrate use requires careful haemodynamic monitoring in this setting
- Left ventricular dysfunction or heart failure – while nitrates can be beneficial, they must be used cautiously to avoid excessive preload reduction
- Low blood pressure or orthostatic hypotension – symptoms such as dizziness when standing up may be worsened by Monoket OD
- Aortic or mitral valve stenosis – narrowing of heart valves may become more symptomatic with nitrate-induced vasodilation
- Raised intracranial pressure – nitrates have historically been considered to potentially increase intracranial pressure, though this effect is debated in current literature
- Hypothyroidism – patients with underactive thyroid may be more sensitive to the cardiovascular effects of nitrates
- Overactive or irritable bowel – the extended-release formulation may pass through the gastrointestinal tract too quickly, before full drug release, resulting in reduced efficacy
Monoket OD and similar nitrate medications can develop tolerance (become less effective) when used continuously at high doses. The once-daily extended-release formulation is specifically designed to provide a nitrate-free interval during the night, which helps prevent this problem. It is therefore essential to follow the prescribed dosing schedule and not take extra doses.
Pregnancy and Breastfeeding
There is limited clinical experience with the use of isosorbide mononitrate during pregnancy. Animal studies have not demonstrated clear evidence of harm, but the potential risk to a human foetus is not well established. If you are pregnant, think you may be pregnant, or are planning to become pregnant, discuss the risks and benefits with your doctor before using Monoket OD.
It is not known whether isosorbide mononitrate passes into breast milk. A risk to the nursing infant cannot be excluded. The necessity of this medication and the benefits of breastfeeding must be weighed against any potential risks to the child. Consult your doctor before using Monoket OD while breastfeeding.
Use in Children
The safety and effectiveness of isosorbide mononitrate in children have not been established. Monoket OD is not recommended for use in the paediatric population.
Driving and Operating Machinery
Monoket OD can cause dizziness, light-headedness, and orthostatic hypotension, particularly at the start of treatment or when the dose is increased. These effects can impair your ability to drive or operate heavy machinery safely. The risk of dizziness is increased if you consume alcohol while taking this medication. You should assess your individual response to the medication before driving and avoid alcohol during treatment.
How Does Monoket OD Interact with Other Drugs?
Monoket OD has several clinically significant drug interactions. The most dangerous is with PDE5 inhibitors (sildenafil, tadalafil, vardenafil), which is strictly contraindicated. It also interacts with other blood pressure–lowering medications, riociguat, and alcohol, all of which can potentiate hypotension.
Drug interactions with Monoket OD are primarily related to its vasodilating and blood pressure–lowering effects. When combined with other medications that also lower blood pressure, the result can be a dangerous and sometimes life-threatening drop in blood pressure (hypotension). Always inform your doctor or pharmacist about all medications you are taking, including prescription drugs, over-the-counter medications, and herbal supplements.
Major Interactions
| Drug / Class | Interaction | Risk Level |
|---|---|---|
| Sildenafil (Viagra) | Life-threatening hypotension via combined cGMP pathway potentiation | Contraindicated |
| Tadalafil (Cialis) | Severe hypotension; tadalafil has long half-life (17.5 hours) extending risk window | Contraindicated |
| Vardenafil (Levitra) | Profound hypotension with potential cardiovascular and cerebral damage | Contraindicated |
| Riociguat (Adempas) | Both drugs increase cGMP; additive hypotensive effect | Contraindicated |
Moderate Interactions
| Drug / Class | Interaction | Clinical Advice |
|---|---|---|
| Beta-blockers (e.g., metoprolol, atenolol) | Additive blood pressure reduction and potential excessive heart rate lowering | Monitor blood pressure; often used together under medical supervision |
| Calcium channel blockers (e.g., amlodipine, diltiazem) | Enhanced hypotensive effect through dual vasodilation mechanisms | Dose adjustment may be needed; monitor for dizziness |
| Other vasodilators | Additive blood pressure reduction | Monitor blood pressure closely |
| Tricyclic antidepressants (e.g., amitriptyline) | May enhance hypotensive effect of nitrates | Monitor for orthostatic hypotension |
| MAO inhibitors | Potential enhancement of blood pressure–lowering effects | Use with caution; close blood pressure monitoring |
| Antipsychotics (neuroleptics) | Additive hypotension due to alpha-blocking properties of many neuroleptics | Monitor for dizziness and falls |
| Dihydroergotamine (migraine medication) | May paradoxically increase blood pressure; reduced antianginal effect of nitrate | Avoid concurrent use if possible |
| Sapropterin (BH4) | Both act on the nitric oxide pathway; potential additive effects | Consult your doctor before combining |
| Alcohol | Enhances the blood pressure–lowering effect; increases dizziness risk | Avoid or limit alcohol consumption |
The interaction between nitrates and PDE5 inhibitors is absolute. You must not discontinue Monoket OD in order to take a PDE5 inhibitor for erectile dysfunction. If you require treatment for erectile dysfunction, speak to your doctor about alternative medications that do not interact with nitrates.
What Is the Correct Dosage of Monoket OD?
The standard dose of Monoket OD is one 50 mg extended-release capsule taken once daily in the morning. Treatment usually begins with 25 mg daily for the first 2–4 days to reduce the risk of headache, then the dose is increased as directed by your doctor.
Adults
Standard Adult Dosage
- Starting dose: 25 mg once daily in the morning for the first 2–4 days (to minimise headache)
- Maintenance dose: 50 mg once daily in the morning
- Dose escalation: May be increased stepwise to 75 mg and up to 100 mg once daily, as directed by your doctor
- Maximum dose: 100 mg once daily
The capsules should be taken at the same time each morning to maintain a consistent nitrate-free interval during the night. This interval is essential to prevent the development of nitrate tolerance, which would reduce the medication's effectiveness. Swallow the capsules whole with at least half a glass of water – do not chew, crush, or open them, as this will destroy the extended-release mechanism and may lead to dose dumping (a rapid release of the entire dose at once).
If you feel that Monoket OD is too strong or too weak, do not adjust the dose yourself. Consult your doctor, who can reassess your dosing based on your clinical response, blood pressure readings, and any side effects you may be experiencing.
Elderly Patients
No specific dose adjustment is routinely required for elderly patients. However, elderly individuals are often more susceptible to the hypotensive effects of nitrates, particularly orthostatic hypotension (dizziness upon standing). Therefore, careful dose titration starting with 25 mg is especially important in this population, and blood pressure should be monitored regularly.
Children
The safety and efficacy of Monoket OD in children have not been established. This medication is not indicated for use in paediatric patients.
Missed Dose
If you forget to take your dose of Monoket OD, take it as soon as you remember on the same day. However, if it is already late in the afternoon or close to bedtime, skip the missed dose entirely and take your next dose the following morning at the usual time. Do not take a double dose to compensate for a missed one. Taking the dose late in the day could disrupt the nitrate-free interval during the night, which is necessary to prevent tolerance.
Overdose
An overdose of isosorbide mononitrate can cause severe and potentially dangerous symptoms due to excessive vasodilation. If you suspect an overdose, contact emergency services or your local poison control centre immediately.
Symptoms of overdose may include:
- Severe headache
- Pale skin (pallor)
- Excessive sweating
- Weak, rapid pulse (tachycardia)
- Nausea and vomiting
- Diarrhoea
- Severe dizziness, especially when standing
- Dangerously low blood pressure (hypotension)
- Fainting or loss of consciousness
If you experience any of these symptoms after taking too much Monoket OD, stop taking the medication and lie down with your legs slightly elevated to help restore blood flow to the brain. Seek immediate medical attention. Do not attempt to manage a suspected overdose at home.
| Patient Group | Starting Dose | Maintenance Dose | Maximum Dose |
|---|---|---|---|
| Adults | 25 mg once daily (2–4 days) | 50 mg once daily | 100 mg once daily |
| Elderly | 25 mg once daily (gradual titration) | 50 mg once daily | 100 mg once daily |
| Children | Not recommended | Not established | Not established |
What Are the Side Effects of Monoket OD?
The most common side effect of Monoket OD is headache, which occurs in more than 1 in 10 patients and typically resolves within the first 1–2 weeks of treatment. Other common side effects include dizziness, nausea, weakness, low blood pressure upon standing, and increased heart rate.
Like all medicines, Monoket OD can cause side effects, although not everyone will experience them. The side effects are primarily related to the medication's vasodilating action. Headache, the most frequent adverse effect, occurs because the nitrate dilates blood vessels in the brain, leading to increased intracranial pressure. This side effect typically diminishes as the body adjusts to the medication. Starting with a lower dose (25 mg) for the first 2–4 days helps minimise its severity.
Stop taking Monoket OD and contact a doctor immediately if you develop swelling of the face, tongue, or throat, difficulty swallowing, hives, or difficulty breathing. These may be signs of angioedema, a serious allergic reaction that requires urgent treatment.
Very Common
May affect more than 1 in 10 people
- Headache (often improves with continued use)
Common
May affect up to 1 in 10 people
- Dizziness and light-headedness
- Nausea
- Weakness and fatigue
- Low blood pressure upon standing (orthostatic hypotension)
- Increased heart rate (tachycardia)
Uncommon
May affect up to 1 in 100 people
- Vomiting
- Diarrhoea
- Worsening of angina symptoms (paradoxical angina)
Rare
May affect up to 1 in 1,000 people
- Allergic skin reactions (rash)
- Flushing (skin redness)
Frequency Not Known
Cannot be estimated from available data
- Angioedema (swelling of face, tongue, or throat)
- Circulatory collapse (sometimes accompanied by slow, irregular heartbeat and fainting)
- Severe hypotension
- Exfoliative dermatitis (peeling skin)
- Drowsiness
- Heartburn
- Hypoxaemia (low blood oxygen levels)
Severe symptoms due to low blood pressure have been reported with nitrate therapy and may include nausea, vomiting, restlessness, pallor (pale skin), and excessive sweating. If you experience any of these symptoms, particularly upon standing, sit or lie down immediately and contact your healthcare provider.
If any side effect becomes bothersome or persists, or if you notice side effects not listed above, please inform your doctor or pharmacist. Reporting suspected adverse reactions after the authorisation of the medicinal product is important, as it allows continued monitoring of the benefit–risk balance of the medication.
How Should You Store Monoket OD?
Store Monoket OD at or below 30°C in its original packaging. Keep it out of the sight and reach of children. Do not use the medication after the expiry date printed on the packaging.
Proper storage of Monoket OD is important to maintain the medication's effectiveness and safety. Follow these storage guidelines:
- Temperature: Store at room temperature, not exceeding 30°C (86°F). Avoid exposing the medication to excessive heat or direct sunlight.
- Children: Keep this medication out of the sight and reach of children at all times.
- Expiry date: Do not use Monoket OD after the expiry date stated on the packaging. The expiry date refers to the last day of the stated month.
- Disposal: Do not dispose of medicines in household waste or via the sewage system. Return unused or expired medications to your pharmacy for safe disposal. This helps protect the environment.
What Does Monoket OD Contain?
Each Monoket OD extended-release capsule contains isosorbide mononitrate as the active substance in strengths of 25 mg, 50 mg, or 100 mg. The capsules also contain lactose monohydrate, sucrose, and other excipients.
The following information details the composition of Monoket OD extended-release capsules:
Active Substance
Isosorbide mononitrate – available in 25 mg, 50 mg, and 100 mg capsule strengths. This is the pharmacologically active component responsible for the medication's vasodilating and anti-anginal effects.
Inactive Ingredients (Excipients)
- Lactose monohydrate (6.7 mg, 13.4 mg, or 26.5 mg per capsule depending on strength) – a commonly used filler in pharmaceutical formulations
- Sucrose – used as a coating agent for controlled drug release
- Maize starch – provides structural support to the capsule contents
- Talc – used as a lubricant and anti-adherent
- Ethylcellulose – a polymer used in the extended-release coating to control drug dissolution
- Macrogol (polyethylene glycol) – plasticiser in the coating
- Hydroxypropylcellulose – binder and film-forming agent
- Gelatin – capsule shell material
- Titanium dioxide (E171) – white pigment used to colour the capsule shell
- Iron oxide (E172) – pigment used for capsule shell colouring
Monoket OD contains both lactose and sucrose. If you have been told by your doctor that you have an intolerance to certain sugars, contact your doctor before taking this medication. Patients with rare hereditary problems of galactose intolerance, total lactase deficiency, or glucose–galactose malabsorption should not take this medicine.
Packaging
Monoket OD is available in blister packs containing 14 or 98 extended-release capsules, depending on the strength. Not all pack sizes may be marketed in all countries.
Frequently Asked Questions About Monoket OD
Monoket OD (isosorbide mononitrate) is used for the long-term prevention of angina pectoris – chest pain caused by reduced blood flow to the heart muscle. It works by dilating blood vessels, which reduces the workload on the heart and improves oxygen supply to the myocardium. It is important to understand that Monoket OD is a preventive medication and does not work fast enough to treat an acute angina attack. For acute chest pain, you should use a fast-acting nitrate such as sublingual nitroglycerin.
No. Monoket OD must never be taken together with PDE5 inhibitors such as sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra). Both medications work through the nitric oxide–cGMP pathway, and their combined use can cause a dangerous and potentially life-threatening drop in blood pressure that may lead to heart attack, stroke, or death. This interaction is absolute – you should not stop taking Monoket OD in order to take a PDE5 inhibitor. If you need treatment for erectile dysfunction, speak with your doctor about alternative options that are safe to use with nitrates.
Headache is the most common side effect of isosorbide mononitrate, affecting more than 1 in 10 patients. It occurs because the medication dilates blood vessels, including those in the brain, which increases intracranial blood flow and pressure. The good news is that this side effect typically diminishes or disappears within the first 1–2 weeks of treatment as the body adjusts. To minimise headache, doctors usually recommend starting with a lower dose (25 mg) for the first 2–4 days before increasing to the full maintenance dose. Over-the-counter painkillers such as paracetamol may help manage the headache during this adjustment period.
If you forget your morning dose, take it as soon as you remember on the same day. However, if it is already late in the afternoon or evening, skip the missed dose and take your next dose the following morning at the usual time. Do not take a double dose to make up for a missed one. Taking Monoket OD late in the day can interfere with the nitrate-free interval that is necessary to maintain the drug's effectiveness and prevent tolerance development.
Yes, this is a well-known phenomenon called nitrate tolerance. When nitrates are given continuously without a break, the blood vessels become less responsive to the medication over time. Monoket OD is specifically formulated as an extended-release once-daily capsule taken in the morning to provide a nitrate-free interval during the night (typically 10–12 hours). This drug-free period allows the vascular system to recover its sensitivity to nitrates. It is therefore essential to follow your prescribed dosing schedule, take the capsule in the morning, and avoid taking extra doses or additional nitrate medications in the evening.
There is limited experience with the use of isosorbide mononitrate during pregnancy, and the potential risk to a developing foetus is not well established. If you are pregnant, think you may be pregnant, or are planning to become pregnant, you should consult your doctor before using Monoket OD. Your doctor will carefully assess whether the benefits of treatment outweigh the potential risks. Similarly, it is not known whether the drug passes into breast milk, so breastfeeding mothers should discuss the situation with their healthcare provider.
References
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- World Health Organization. WHO Model List of Essential Medicines – 23rd list, 2023. Geneva: WHO; 2023.
- Thadani U, Lipicky RJ. Short and long-acting oral nitrates for stable angina pectoris. Cardiovascular Drugs and Therapy. 1994;8(4):611–623.
- Parker JD, Parker JO. Nitrate therapy for stable angina pectoris. New England Journal of Medicine. 1998;338(8):520–531.
- Münzel T, Daiber A, Gori T. Nitrate therapy: new aspects concerning molecular action and tolerance. Circulation. 2011;123(19):2132–2144.
- National Institute for Health and Care Excellence (NICE). Stable angina: management. Clinical guideline [CG126]. 2016 (updated).
- British National Formulary (BNF). Isosorbide mononitrate. London: BMJ Group and Pharmaceutical Press; 2025.
- European Medicines Agency (EMA). Summary of Product Characteristics – Isosorbide mononitrate. Accessed 2025.
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