Class Ib antiarrhythmic / sodium channel blocker Pregnancy: There are no or limited data from the use of mexiletine in pregnant women; limited clinical data show that mexiletine crosses the placenta and reaches the foetus. Animal studies do not indicate reproductive toxicity. As a precautionary measure, it is preferable to avoid the use of mexiletine during pregnancy. Mexiletine is excreted in human milk with insufficient information on effects in newborns/infants — decide whether to discontinue breast-feeding or to discontinue/abstain from mexiletine therapy.

Mexiletine

Brand names: Namuscla

Mexiletine is an oral class Ib antiarrhythmic used in the management of symptomatic ventricular arrhythmias, and is also used in some non-dystrophic myotonias.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Maintenance 150 mg to 300 mg, two to three times daily. In patients in whom rapid control of ventricular arrhythmia is needed, a loading dose of 400 mg may be given

Route: Oral (capsules swallowed whole with ample liquid, preferably with the patient in an upright position; advisable to take with food to minimise gastrointestinal adverse effects)

Frequency: Two to three times daily

Max: Dosage should not exceed 1200 mg per day

Source: Mexiletine hydrochloride 100 mg hard capsules SPC section 4.2. Treatment should be initiated and monitored by a specialist experienced in the treatment of cardiac arrhythmias; the optimal dosage should be determined individually based on the patient's response and tolerance. If necessary the dose may be adjusted in 50 or 100 mg increments, with a minimum of two to three days between dose adjustments. Section 4.4: considering the pro-arrhythmic potential of mexiletine and the lack of evidence of improved survival for class I antiarrhythmic agents in patients without life-threatening arrhythmias, use should be reserved for patients with life-threatening ventricular arrhythmia; haematological monitoring (haemogram including WBC differential and platelet count) should be performed before initiation and during therapy. Hepatic impairment: exercise caution in mild or moderate impairment due to the potential for higher plasma exposure, allowing a minimum of two weeks between dose adjustments; mexiletine should not be used in severe hepatic impairment. Poor CYP2D6 metabolisers (7% of the European population): potential for increased plasma levels — allow a minimum of one week between dose adjustments. Elderly: no dosage adjustment is required for older patients with normal renal function. Paediatric population: the safety and efficacy of mexiletine hard capsules in children and adolescents aged 0 to 18 years have not yet been established; no data are available. US labelling (for comparison only, not used for the dose above): initiate with 200 mg every eight hours when rapid control is not essential, satisfactory control in most patients at 200 to 300 mg every eight hours with food or antacid, 400 mg every eight hours may be tried if response is inadequate and tolerance is good, not exceeding 1200 mg/day.

Dose adjustments

Renal

No dosage adjustment is considered necessary in patients with mild or moderate renal impairment. There is no clinical experience in patients with severe renal impairment (creatinine clearance < 30 ml/min), therefore mexiletine should not be used in these patients (section 4.2).

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to mexiletine hydrochloride or to local anaesthetics of amide type
Hypersensitivity to any of the excipients
Sinus node dysfunction (unless a pacemaker is present)
Severe atrioventricular (AV) conduction disturbances (unless a pacemaker is present)
Severe heart failure; cardiogenic shock

Side effects

Dizziness, tremor, headache, paraesthesia, blurred vision, numbness
Abdominal pain and dyspepsia (very common/common); nausea, constipation, dry mouth
Tachycardia, palpitations, angina pain, atrial fibrillation, bradycardia
Fatigue, asthenia, chest discomfort, malaise, ataxia
Neutropenia, agranulocytosis, leukopenia, thrombocytopenia; drug reaction with eosinophilia and systemic symptoms (DRESS)
Abnormal hepatic function / abnormal liver function tests

Interactions

CYP2D6 inhibitors — high mexiletine plasma levels may be observed in patients taking medicinal products that inhibit CYP2D6; if necessary, a dose increase is recommended only after a period of at least 7 days (section 4.4, cross-referencing section 4.5)
Cigarette smoking — mexiletine pharmacokinetics are affected by smoking and doses may need to be increased or decreased if patients start or stop smoking (section 4.4)
NOTE: section 4.5 was not captured in the fetched source — review it in full before publication

Clinical monograph

How it works

It blocks fast sodium channels, preferentially in active or depolarised tissue, shortening the action potential duration and reducing abnormal myocardial and muscle membrane excitability.

Prescribing in practice

Like other antiarrhythmics it is proarrhythmic and should be used with caution under specialist guidance, particularly with bradyarrhythmias, conduction disturbance or significant heart failure.
Gastrointestinal and central nervous system effects such as nausea, tremor and dizziness are common and may limit treatment.
Plasma concentrations are affected by hepatic enzyme activity and by changes in urinary pH; consult the SPC for cautions and interactions.

Monitoring

Monitor the ECG and clinical response, with assessment of liver function and consideration of plasma-level checks in selected patients.

Counselling the patient

Advise taking the medicine with food to reduce stomach upset.
Tell the patient to report fainting, palpitations or new rashes promptly.

Evidence & guidelines

Mexiletine is an established class Ib antiarrhythmic; refer to the SPC and current prescribing references for indications and cautions.

Reference: NICE; SmPC; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways