- Drugs List
- Therapeutic Indications
- Precautions and Warnings
- Pregnancy and Lactation
- Side Effects
Oral formulations of mexiletine.
Life-threatening ventricular arrhythmias
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.
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.
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
Congestive cardiac failure
CYP2D6 poor metaboliser genotype
First degree atrioventricular block - if treating ventricular arrhythmias
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.
Abnormal liver function
Abnormal liver function tests
Acute hepatic injury
Drug rash with eosinophilia and systemic symptoms (DRESS)
Loss of consciousness (transient)
Lupus erythematosus-like syndrome
Reversible confusional states
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 ( www.toxbase.org ) or if this is unavailable at the backup site ( www.toxbasebackup.org ).
Last Full Review Date: April 2022
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: www.nice.org.uk Last accessed: 05 April 2022.
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Medscape UK | Univadis prescription drug monographs & interactions are based on FDB Multilex Content
FDB Disclaimer : FDB Multilex is intended for the use of healthcare professionals and is provided on the basis that the healthcare professionals will retain FULL and SOLE responsibility for deciding what treatment to prescribe or dispense for any particular patient or circumstance.