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Mexiletine oral


Oral formulations of mexiletine.

Drugs List

  • mexiletine hydrochloride 100mg capsules
  • mexiletine hydrochloride 200mg capsules
  • mexiletine hydrochloride 50mg capsules
  • NAMUSCLA 167mg capsules
  • Therapeutic Indications


    Life-threatening ventricular arrhythmias
    Non-dystrophic myotonia

    Symptomatic treatment of myotonia in adult patients with non-dystrophic myotonic disorders.
    Treatment of ventricular arrhythmias which are considered life-threatening.


    Regular assessment of treatment should be implemented. Long term treatment should not be continued in patients not responding or benefiting from treatment.


    Myotonia in adult patients with non-dystrophic myotonic disorder
    Doses given below are in relation to the strength of the base mexiletine.
    167mg daily.

    After at least 1 week of initial treatment of 167mg daily, the dose can be increased to 333mg daily based on clinical response.
    After a further week, the dose can be increased to 500mg daily based on clinical response.

    Maintenance treatment of 167mg to a maximum of 500mg daily should be determined by the intensity of symptoms and clinical response.
    Treatment of potentially life-threatening ventricular arrhythmias
    Loading dose 400mg may be given to patients in need of rapid control of ventricular arrhythmias.
    Maintenance dose
    150mg to 300mg, two to three times daily. Dose may be adjusted in increments of 50mg or 100mg with a minimum of two or three days between dose adjustments. Maximum total daily dose is 1200mg.

    Patients with Hepatic Impairment

    Due to the potential for higher plasma exposure, patients with mild or moderate hepatic impairment should leave a minimum of two weeks between dose adjustments.

    Additional Dosage Information

    Patients who are CYP2D6 poor metabolisers may exhibit higher mexiletine blood levels. Leave at least 7 days before a dose increase to ensure steady levels of mexiletine are reached.


    Children under 18 years
    Atrial fibrillation - if treating myotonia
    Atrial flutter - if treating myotonia
    Atrial tachyarrhythmia - if treating myotonia
    Cardiogenic shock - if treating ventricular arrhythmias
    Heart block (complete/risk of evolving to complete) - if treating myotonia
    Heart failure with ejection fraction <50% - if treating myotonia
    History of myocardial infarction - if treating myotonia
    Inherited long QT syndrome - if treating ventricular arrhythmias
    Left ventricular ejection fraction <55%- if treating ventricular arrhythmia
    Non-paced severe AV conduction defect - if treating ventricular arrhythmia
    Non-paced sinus node dysfunction - if treating ventricular arrhythmias
    Severe cardiac failure - if treating ventricular arrhythmias
    Severe hepatic disorder
    Severe renal impairment
    Sinus node dysfunction - if treating myotonia
    Symptomatic coronary artery disease - if treating myotonia
    Ventricular tachycardia - if treating myotonia

    Precautions and Warnings

    Tobacco smoking
    Cardiac disorder
    Congestive cardiac failure
    CYP2D6 poor metaboliser genotype
    Epileptic disorder
    First degree atrioventricular block - if treating ventricular arrhythmias
    Hepatic impairment
    History of seizures
    Intraventricular conduction defects - if treating ventricular arrhythmias

    Correct electrolyte disorders before treatment
    Advise ability to drive/operate machinery may be affected by side effects
    Not all available brands are licensed for all indications
    Treatment to be initiated and supervised by a specialist
    Administer with the patient in an upright position
    Monitor cardiac function before and periodically during treatment
    Monitor differential WBC count before and during therapy
    Monitor platelets before starting and during treatment
    Monitor serum electrolytes before and during treatment
    Monitor cardiac function before and after dose increase
    Monitor hepatic enzymes
    Monitor patients for signs and symptoms of cardiac failure
    Monitor patients with epilepsy while taking this treatment
    Advise patient to report any signs of cardiac arrhythmias
    Counsel patients on symptoms of AV block or arrhythmias
    Discontinue if any deterioration in cardiac status occurs
    Discontinue if haematological abnormalities develop
    Discontinue if hepatic enzymes (AST or ALT) become persistently raised
    Dose adjustment required if patient starts/stops smoking during therapy
    Advise patient to avoid excessive caffeine
    Remind patient of importance of carrying Alert Card with them at all times

    Mexiletine may induce arrhythmia or accentuate a pre-existing undiagnosed or diagnosed arrhythmia. Before starting treatment, all patients should undergo comprehensive cardiac evaluation.

    For the treatment of myotonia in adults with non-dystrophic myotonia, cardiac monitoring during treatment should be adapted in line with cardiac function of the patient as follows:
    In patients without cardiac abnormalities, periodic ECG monitoring is recommended every 2 years or more if considered necessary.
    In patients with cardiac abnormalities or patients prone to abnormalities, detailed cardiac evaluation should be performed before and after any dose increase. Additionally during maintenance treatment, cardiac evaluation is recommended at least annually or more frequently if considered necessary.

    Mexiletine dose may need to be increased if a patient starts to smoke. If a patient stops smoking during treatment then the dose may need to be reduced.

    Haematologic monitoring is advised prior to, and throughout treatment with mexiletine. If significant changes in haematologic parameters occurs, discontinuation should be considered. Blood counts are expected to return to normal within one month of discontinuation.

    Whilst taking mexiletine, dietary regimens or concomitant drug therapy which substantially changes urinary pH should be avoided.

    Pregnancy and Lactation


    Mexiletine is contraindicated during pregnancy.

    Use of mexiletine during pregnancy is contraindicated by the manufacturer. At the time of writing there is limited published information regarding the use of mexiletine during pregnancy. However, available reports indicate that mexiletine crosses the placenta. Potential risks are unknown.


    Mexiletine is contraindicated during breastfeeding.

    Use of mexiletine when breastfeeding is contraindicated by the manufacturer. Mexiletine is known to be excreted into breast milk. Briggs (2015) states that the concentration of mexiletine excreted into breast milk exceeds that in maternal serum. The manufacturer states that a decision should be made whether to discontinue breastfeeding or to discontinue mexiletine therapy, taking into account the risks and benefits for both the mother and infant. Effects on exposed infants are unknown.


    To be taken during a meal.
    Swallow capsule with plenty of water while sitting or standing, avoiding a supine position.
    Avoid excessive caffeine intake during treatment.
    Advise patient on the presenting symptoms of cardiac arrhythmias and to report any signs of cardiac arrhythmias immediately.
    Advise ability to drive or operate machinery may be affected by side effects.

    Side Effects

    Abdominal pain
    Abnormal liver function
    Abnormal liver function tests
    Acute hepatic injury
    Anginal pain
    Atrial fibrillation
    Atrioventricular block
    Blurred vision
    Cardiac failure
    Chest discomfort
    Circulatory collapse
    Drug rash with eosinophilia and systemic symptoms (DRESS)
    Dry mouth
    Dry skin
    Exfoliative dermatitis
    Hepatic damage
    Hepatic disorders
    Hepatic necrosis
    Hot flushes
    Loss of consciousness (transient)
    Lupus erythematosus-like syndrome
    Oesophageal perforation
    Oesophageal ulceration
    Painful extremities
    Pulmonary fibrosis
    Reversible confusional states
    Speech disturbances
    Stevens-Johnson syndrome


    It is strongly recommended that the UK National Poisons Information Service be consulted on cases of suspected or actual overdose where there is doubt over the degree of risk or about appropriate management.

    The following number will direct the caller to the relevant local centre (0844) 892 0111

    Information may be obtained if you have access to ToxBase the primary clinical toxicology database of the National Poisons Information Service. This is available via password on the internet ( ) or if this is unavailable at the backup site ( ).

    Further Information

    Last Full Review Date: April 2022

    Reference Sources

    Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk, 10th edition (2015) ed. Briggs, G., Freeman, R. Wolters Kluwer Health, Philadelphia.

    Summary of Product Characteristics: Mexiletine hydrochloride 50mg, 100mg and 200mg Hard Capsules. Clinigen Healthcare Ltd. Revised August 2021.

    Summary of Product Characteristics: Namuscla 167mg hard capsules. Lupin Europe GmbH. Revised December 2018.

    NICE Evidence Services Available at: Last accessed: 05 April 2022.

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