Namuscla: Uses, Dosage & Side Effects

What Is Namuscla and What Is It Used For?

Namuscla contains the active substance mexiletine hydrochloride, a sodium channel blocker that has been used in clinical practice for several decades, originally as a Class IB antiarrhythmic agent for the treatment of ventricular arrhythmias. In 2018, the European Medicines Agency (EMA) granted marketing authorization for Namuscla specifically for the symptomatic treatment of myotonia in adult patients with non-dystrophic myotonic disorders, making it the first medicine to receive formal approval for this orphan indication. This approval addressed a significant gap in the therapeutic landscape, as previously mexiletine could only be obtained through off-label prescribing or compassionate use programs in many countries.

Non-dystrophic myotonic disorders (NDMs) are a group of rare inherited neuromuscular conditions caused by mutations in genes encoding skeletal muscle ion channels. The most common forms include myotonia congenita (caused by mutations in the CLCN1 gene encoding the chloride channel ClC-1, with autosomal dominant Thomsen disease and autosomal recessive Becker disease being the two subtypes), paramyotonia congenita (caused by gain-of-function mutations in the SCN4A gene encoding the sodium channel Nav1.4), and sodium channel myotonia (also caused by SCN4A mutations). These conditions share the cardinal symptom of myotonia: involuntary sustained muscle contraction followed by delayed relaxation that causes stiffness, difficulty initiating movement, and in some cases pain. The prevalence of non-dystrophic myotonic disorders is estimated at approximately 1 in 100,000 individuals, though this figure likely represents an underestimate due to underdiagnosis.

The clinical impact of myotonia on patients' daily lives can be substantial. Myotonic stiffness typically affects the hands, legs, and facial muscles, making it difficult to open the hand after gripping an object, to begin walking after standing still, or to speak clearly after a period of silence. Some patients experience a characteristic "warm-up" phenomenon in which repeated voluntary muscle contractions gradually reduce the severity of myotonia, while others (particularly those with paramyotonia congenita) experience paradoxical worsening of stiffness with repeated activity, especially in cold environments. Falls are common due to leg stiffness, and many patients report significant limitations in occupational, recreational, and social activities.

Mexiletine exerts its antimyotonic effect by blocking voltage-gated sodium channels (Nav1.4) in skeletal muscle. In non-dystrophic myotonic disorders, the fundamental pathophysiology involves abnormal ion channel function that leads to increased sodium conductance across the muscle cell membrane. In sodium channel myotonias and paramyotonia congenita, gain-of-function mutations in SCN4A cause the sodium channel to remain open for longer than normal or to recover from inactivation too quickly, resulting in sustained depolarization and repetitive firing of action potentials (myotonic runs). In chloride channel myotonias (myotonia congenita), loss-of-function mutations in CLCN1 reduce the chloride conductance that normally stabilizes the resting membrane potential, making the muscle fiber membrane hyperexcitable and susceptible to self-sustaining bursts of action potentials. In both scenarios, mexiletine provides therapeutic benefit by blocking the persistent sodium current, thereby stabilizing the muscle membrane and reducing the frequency and duration of myotonic discharges.

The clinical efficacy of mexiletine in non-dystrophic myotonia was established in the MYOMEX trial, a pivotal randomized, double-blind, placebo-controlled, crossover study published in JAMA in 2012. This trial enrolled 59 patients with genetically confirmed non-dystrophic myotonia and demonstrated that mexiletine significantly reduced clinical myotonia severity, as measured by a validated stiffness Visual Analogue Scale (VAS), compared with placebo. The mean decrease in stiffness VAS was 2.89 points (on a 0–10 scale) greater with mexiletine than with placebo (p < 0.001). Secondary endpoints, including the Clinical Myotonia Rating Scale, grip force relaxation time, and eye-closure time, also showed statistically significant improvements. The onset of therapeutic effect was rapid, with most patients reporting improvement within the first week of treatment.

What Should You Know Before Taking Namuscla?

Contraindications

Namuscla must not be used in several clinical scenarios due to serious safety risks. The absolute contraindications to Namuscla treatment include hypersensitivity to mexiletine hydrochloride, to any of the excipients, or to any local anesthetic of the amide type (such as lidocaine). Because mexiletine is structurally related to lidocaine and shares similar pharmacological properties, patients who have experienced allergic reactions to lidocaine or similar agents should not use Namuscla.

Namuscla is strictly contraindicated in patients with ventricular tachyarrhythmias, as mexiletine's sodium channel-blocking activity can worsen pre-existing ventricular arrhythmias or precipitate life-threatening cardiac events. Patients with second- or third-degree atrioventricular (AV) block who do not have a pacemaker must not use Namuscla, because the drug can further impair cardiac conduction and lead to complete heart block or cardiac arrest. Similarly, Namuscla is contraindicated in patients with heart failure with reduced ejection fraction (NYHA class III or IV), as the negative inotropic effect of mexiletine can worsen cardiac function in these patients. Patients with sinus node dysfunction should also not use Namuscla without a pacemaker in place.

Namuscla is contraindicated during pregnancy. Animal reproductive toxicity studies have demonstrated adverse effects on fetal development, including an increased incidence of fetal abnormalities and reduced fetal viability, at doses within or near the human therapeutic range. Mexiletine crosses the placental barrier, and adequate human data are not available to establish safety during pregnancy. Women of childbearing potential must use effective contraception during treatment and for at least one month after discontinuation.

Warnings and Precautions

The most critical safety concern with Namuscla is its potential for cardiac adverse effects. As a Class IB antiarrhythmic agent, mexiletine can cause proarrhythmic effects, meaning it can paradoxically worsen existing cardiac arrhythmias or trigger new ones. This risk is heightened in patients with underlying structural heart disease, electrolyte imbalances (particularly hypokalemia and hypomagnesemia), or those taking other medications that affect cardiac conduction or QT interval. Before initiating Namuscla, a thorough cardiac evaluation including 12-lead ECG should be performed to rule out conduction abnormalities, pre-existing arrhythmias, or QT prolongation.

Hepatotoxicity has been reported with mexiletine use, including rare cases of severe liver injury. Liver function tests (transaminases, bilirubin, alkaline phosphatase) should be performed before starting treatment and periodically thereafter, particularly during the first months of therapy. If liver enzyme elevations exceed three times the upper limit of normal or if clinical signs of liver injury develop (jaundice, dark urine, right upper quadrant pain, persistent nausea, unexplained fatigue), Namuscla should be discontinued immediately and liver function monitored until normalization.

Blood dyscrasias, including leukopenia, agranulocytosis, and thrombocytopenia, have been reported rarely with mexiletine. Complete blood counts should be monitored periodically. Patients should be advised to report signs of infection (fever, sore throat, mouth ulcers) or unusual bleeding or bruising, which may indicate hematological abnormalities.

• Epilepsy: Mexiletine may lower the seizure threshold in patients with epilepsy or a history of seizures. Use with caution and monitor closely.
• Hepatic impairment: Mexiletine is extensively metabolized by the liver. In patients with moderate to severe hepatic impairment, plasma levels may be significantly elevated, increasing the risk of toxicity. Dose reduction and close monitoring are required.
• Renal impairment: Although renal excretion plays a minor role in mexiletine elimination, caution is advised in patients with severe renal impairment, as accumulation of metabolites may occur.
• CYP2D6 poor metabolizers: Approximately 7–10% of the Caucasian population are poor metabolizers of CYP2D6 substrates. These individuals may have significantly higher plasma levels of mexiletine, increasing the risk of adverse effects. Dose adjustment may be necessary.

Pregnancy and Breastfeeding

Namuscla is contraindicated during pregnancy due to reproductive toxicity observed in animal studies. In rat and rabbit studies, mexiletine administration during organogenesis was associated with increased fetal mortality, reduced fetal body weight, and skeletal and visceral abnormalities at doses that produced maternal plasma levels comparable to those achieved at therapeutic doses in humans. There are no adequate and well-controlled studies of mexiletine use during human pregnancy. If a patient becomes pregnant while taking Namuscla, the drug should be discontinued immediately and the patient informed of the potential risk to the fetus. Women of childbearing potential should be advised to use effective contraception during treatment and for at least one month after the last dose.

Mexiletine is excreted in human breast milk, with concentrations in breast milk reported to be higher than or comparable to those in maternal plasma. The potential for serious adverse effects in the breastfed infant, including cardiac and neurological effects, cannot be excluded. Therefore, breastfeeding is not recommended during Namuscla treatment and for at least two days after the last dose (corresponding to approximately five elimination half-lives). The decision to discontinue breastfeeding or to discontinue Namuscla treatment should take into account the benefit of breastfeeding for the infant and the clinical benefit of Namuscla for the mother.

Driving and Operating Machinery

Namuscla may have a minor influence on the ability to drive and use machines. Dizziness, visual disturbances, and fatigue have been reported as side effects and could impair the ability to drive or operate machinery. Patients should be advised to assess their individual response to Namuscla before driving or engaging in activities requiring alertness and coordination. If these adverse effects occur, patients should refrain from driving or operating machinery until the symptoms resolve.

How Does Namuscla Interact with Other Drugs?

Unlike monoclonal antibody therapies, mexiletine undergoes extensive hepatic metabolism and has a clinically significant drug interaction profile. Approximately 90% of mexiletine is metabolized in the liver, primarily through the cytochrome P450 enzymes CYP2D6 and CYP1A2, with minor contributions from CYP3A4. This means that drugs which inhibit or induce these enzymes can substantially alter mexiletine plasma concentrations, potentially leading to either toxicity (from elevated levels) or loss of efficacy (from reduced levels). Understanding these interactions is essential for safe and effective Namuscla therapy.

CYP1A2 is the primary metabolic pathway for mexiletine at steady state. Potent CYP1A2 inhibitors such as fluvoxamine, ciprofloxacin, enoxacin, and cimetidine can significantly increase mexiletine plasma concentrations, raising the risk of dose-dependent adverse effects including cardiac toxicity and gastrointestinal symptoms. Co-administration with potent CYP1A2 inhibitors should be avoided when possible; if co-administration is necessary, Namuscla dose reduction and close clinical monitoring (including ECG) are required. Conversely, CYP1A2 inducers such as cigarette smoking, omeprazole, and rifampicin can accelerate mexiletine metabolism and reduce its plasma levels, potentially diminishing its antimyotonic effect. Patients who start or stop smoking during Namuscla treatment may require dose adjustment.

CYP2D6 also plays an important role in mexiletine metabolism. Co-administration with potent CYP2D6 inhibitors such as paroxetine, fluoxetine, quinidine, and bupropion can increase mexiletine exposure. The magnitude of this interaction is particularly pronounced in CYP2D6 extensive metabolizers (the majority of the population), in whom CYP2D6 contributes significantly to overall mexiletine clearance. In CYP2D6 poor metabolizers (approximately 7–10% of Caucasians), the impact of CYP2D6 inhibitors is minimal because CYP1A2 becomes the predominant metabolic pathway.

Beyond pharmacokinetic interactions, mexiletine has pharmacodynamic interactions with other drugs that affect cardiac electrophysiology. Concomitant use with other Class I antiarrhythmic agents (such as flecainide, propafenone, or quinidine) should be avoided due to the risk of additive sodium channel blockade, which can lead to severe conduction disturbances, heart block, or ventricular arrhythmias. Similarly, the combination of mexiletine with drugs that prolong the QT interval (including certain antipsychotics, macrolide antibiotics, and fluoroquinolones) warrants careful ECG monitoring and clinical assessment of the risk-benefit ratio.

Drugs that affect gastric pH can alter the absorption of mexiletine. Agents that increase gastric pH (such as proton pump inhibitors and antacids) may slightly reduce mexiletine absorption, while drugs that lower gastric pH may increase absorption. Narcotic analgesics (opioids) can slow gastric emptying and delay mexiletine absorption. These interactions are generally of modest clinical significance but should be considered when evaluating unexplained changes in therapeutic response or adverse effects.

What Is the Correct Dosage of Namuscla?

Namuscla should always be used exactly as your doctor has prescribed. The dosage is individualized based on each patient's clinical response and tolerability, using a gradual titration approach to minimize the risk of adverse effects, particularly cardiac and gastrointestinal side effects. Treatment should be initiated and supervised by a physician experienced in the management of myotonic disorders, ideally a neurologist with expertise in neuromuscular diseases.

Adults

The recommended dosing schedule for Namuscla follows a stepwise titration protocol:

The capsules should be swallowed whole with a glass of water and taken with or after food to reduce gastrointestinal side effects. Dose increases should occur at intervals of at least one week, and an ECG must be performed before each dose increase to check for any new conduction abnormalities or arrhythmias. The lowest effective dose should be used for maintenance treatment. Many patients achieve adequate symptom control at doses of 167 mg or 334 mg per day, and dose escalation to 501 mg should only occur if lower doses provide insufficient benefit.

The efficacy of Namuscla should be reassessed regularly by the treating physician. The EMA recommends that treatment benefit be reassessed at least annually. If adequate symptom control has been maintained, a trial dose reduction or temporary discontinuation may be considered to confirm ongoing need for treatment. When discontinuing Namuscla, gradual dose reduction is recommended rather than abrupt cessation, particularly in patients who have been on higher doses or prolonged treatment.

Children and Adolescents

Namuscla is not recommended for use in children and adolescents under 18 years of age. The safety and efficacy of mexiletine have not been established in this age group for the treatment of non-dystrophic myotonia, and no clinical data are available. The pharmacokinetics of mexiletine in the pediatric population have not been adequately characterized, and appropriate dosing cannot be determined. If myotonia treatment is needed in a pediatric patient, the treating neurologist should consider alternative therapeutic approaches or specialist referral.

Elderly Patients

Limited data are available regarding the use of Namuscla in patients aged 65 years and older. Elderly patients may be more susceptible to the cardiac and neurological adverse effects of mexiletine due to age-related changes in cardiac conduction, hepatic metabolism, and renal function. Dose selection should be cautious, starting at the lower end of the dosing range, and patients should be monitored more frequently, including regular ECG assessments. Particular attention should be given to electrolyte status (potassium, magnesium), as electrolyte imbalances are more common in the elderly and can increase the proarrhythmic risk of mexiletine.

Hepatic Impairment

Mexiletine is extensively metabolized by the liver, and patients with hepatic impairment may have significantly elevated plasma levels. In patients with mild hepatic impairment, no dose adjustment is necessary, but close monitoring is recommended. In patients with moderate hepatic impairment, the dose should be reduced and plasma levels monitored if feasible. Namuscla is not recommended in patients with severe hepatic impairment due to the lack of pharmacokinetic data and the high risk of drug accumulation and toxicity.

Missed Dose

If you forget to take a dose of Namuscla, take it as soon as you remember, unless it is almost time for your next scheduled dose. In that case, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a forgotten dose. If you miss more than one dose, resume your usual dosing schedule at the next planned time. If you are unsure about when to take your next dose, contact your doctor or pharmacist for advice.

Overdose

Mexiletine overdose is a medical emergency and requires immediate hospital treatment. Symptoms of overdose may include nausea, vomiting, drowsiness, confusion, bradycardia (slow heart rate), hypotension (low blood pressure), cardiac conduction disturbances, seizures, and in severe cases, cardiac arrest. There is no specific antidote for mexiletine poisoning. Treatment is supportive and symptomatic, with continuous cardiac monitoring, intravenous fluids for hypotension, anticonvulsants for seizures, and temporary cardiac pacing for severe bradycardia or heart block. Alkalinization of the urine may enhance mexiletine excretion. If you suspect an overdose, call your local emergency services or go to the nearest emergency department immediately.

What Are the Side Effects of Namuscla?

Like all medicines, Namuscla can cause side effects, although not everybody gets them. The safety profile of mexiletine has been well characterized over decades of clinical use, both as an antiarrhythmic agent and more recently for the treatment of myotonia. In the pivotal MYOMEX clinical trial and subsequent post-marketing surveillance, the most frequently reported adverse reactions were gastrointestinal in nature, occurring in a significant proportion of patients. Understanding the potential side effects and their frequency helps patients and healthcare providers make informed treatment decisions and recognize adverse reactions early.

Gastrointestinal side effects are the most common reason for dose reduction or treatment discontinuation with Namuscla. These effects are thought to result from mexiletine's local irritant effect on the gastric mucosa and its influence on gastrointestinal smooth muscle. Taking Namuscla with food significantly reduces the incidence and severity of nausea and other gastrointestinal symptoms. If gastrointestinal side effects persist despite taking the medication with food, a dose reduction or slower dose titration schedule may be helpful.

Cardiac side effects, while less common, are the most clinically significant adverse reactions associated with mexiletine. As a Class IB antiarrhythmic, mexiletine can cause proarrhythmic effects in susceptible individuals. New or worsened ventricular arrhythmias, conduction disturbances, and QT prolongation have been reported. These risks underscore the importance of baseline and periodic ECG monitoring during Namuscla therapy.

If you experience any side effects, including any not listed above, tell your doctor or pharmacist. You can also report side effects directly via your national pharmacovigilance system. Reporting side effects helps provide more information about the safety profile of Namuscla and may help identify previously unrecognized adverse reactions. Serious adverse reactions, particularly those involving the heart (palpitations, syncope, chest pain), liver (jaundice, dark urine), or blood (unexplained fever, sore throat, unusual bleeding), should be reported to your doctor immediately and warrant prompt medical evaluation.

How Should You Store Namuscla?

Namuscla hard capsules should be stored at temperatures not exceeding 30°C (86°F). There is no requirement for refrigeration. The capsules should be kept in their original packaging (blister packs within the outer carton) to protect them from moisture and light. Do not remove capsules from the blister pack until immediately before taking them, as exposure to humidity can affect the stability of the formulation.

Keep Namuscla out of the sight and reach of children. Mexiletine overdose can be particularly dangerous in children due to their lower body weight and increased sensitivity to the cardiac effects of sodium channel blockers. If a child accidentally ingests Namuscla capsules, seek emergency medical attention immediately, even if the child appears well, as cardiac symptoms may be delayed.

Do not use Namuscla after the expiry date stated on the carton and blister pack. The expiry date refers to the last day of that month. Do not use the medicine if you notice any visible signs of deterioration, such as discoloration, damage to capsules, or unusual odor.

Do not throw away any medicines via wastewater or household waste. Ask your pharmacist how to throw away medicines you no longer use. Proper disposal of unused medications is important for environmental protection, as pharmaceutical residues entering waterways can have adverse effects on aquatic ecosystems. Many countries have take-back programs or pharmacy-based collection systems for unused medications.

What Does Namuscla Contain?

The active substance in Namuscla is mexiletine hydrochloride. Each hard capsule contains 167 mg of mexiletine hydrochloride, which is equivalent to approximately 138 mg of mexiletine base. Mexiletine hydrochloride is a white to off-white crystalline powder that is freely soluble in water. Its chemical name is 1-(2,6-dimethylphenoxy)-2-aminopropane hydrochloride, and it has the molecular formula C₁₁H₁₇NO·HCl with a molecular weight of 215.72 g/mol.

The other ingredients (excipients) in Namuscla include:

• Capsule contents: Microcrystalline cellulose (filler to ensure uniform capsule volume), colloidal anhydrous silica (anti-caking agent), magnesium stearate (lubricant to facilitate manufacturing), and corn starch (disintegrant to help the capsule contents dissolve).
• Capsule shell: Gelatin (structural component of the capsule shell), titanium dioxide (E171, white pigment), red iron oxide (E172, colorant), and yellow iron oxide (E172, colorant). These components give the capsule its characteristic appearance.
• Printing ink: Shellac, black iron oxide (E172), propylene glycol, and potassium hydroxide. These are used for the identification printing on the capsule.

Namuscla capsules are hard gelatin capsules with a dark orange/brown cap and a light orange body, with "167" printed on the body. They are available in packs of 30 and 200 capsules. Not all pack sizes may be marketed in all countries. The capsules are packaged in PVC/PVDC-aluminium blisters to protect the contents from moisture and ensure shelf stability throughout the product's shelf life.

If you have known allergies or intolerances to any of the listed excipients, inform your healthcare provider before starting Namuscla. In particular, patients with gelatin allergy (rare but documented) should discuss alternative treatment options with their doctor.