Mexiletine

What is mexiletine?

Mexiletine was originally a medication used to treat heart rhythm problems by regulating electrical activity in the heart. It belongs to a family of medications called antiarrhythmics. It may also be used to treat myotonia, a condition where muscles have trouble relaxing after they contract, which can cause stiffness or make it hard to release a grip.

You may hear mexiletine called by one of its brand names, Mexitil. Mexiletine comes as a capsule to be taken by mouth. Mexiletine can come in a liquid form if it is specially prepared by a compounding pharmacy. Not all pharmacies are able to prepare this.

Before giving mexiletine to your child

• Allergies: Let your child’s health-care provider know if they are allergic to mexiletine, lidocaine, any other medications or any ingredients in mexiletine. Your pharmacist can give you a list of the ingredients if needed.
• Other medications: Let your child’s health-care provider know about any other medications, vitamins, supplements or herbal products your child is taking or planning to take. They may need to adjust dosages or monitor your child for side effects.
• Interactions: Be aware of over-the-counter products and always discuss any new medications with your child’s health-care provider.
• Medical history: Tell your child’s health-care provider if they have or have had any heart conditions, low blood pressure, liver disease or seizures.
• Surgery: Tell your child’s health-care provider or dentist that they are taking mexiletine if they need any type of surgery, including dental procedures.
• Drowsiness and dizziness: Mexiletine may cause dizziness or light-headedness, so if your child is old enough to drive, they should avoid driving or operating heavy machinery until they know how mexiletine affects them.
• Smoking and vaping: Let your child’s health-care provider know if your child uses tobacco, or tobacco-like products, as they may make mexiletine less effective.

How to give mexiletine to your child

• Give mexiletine exactly as your child’s health-care provider or pharmacist tells you, even if your child seems better.
• Talk to your child's health-care provider before you stop giving mexiletine for any reason. 
• Give mexiletine at the same time(s) every day. 
• Do not suddenly stop giving this medicine to your child. Talk with your child's health-care provider before you stop giving this medicine for any reason. 
• If giving the capsule, have your child swallow the capsule whole. If they cannot, the capsules can be opened and the contents mixed with a small amount of liquid or soft food and given right away. 
• If giving the liquid, measure the appropriate dose with an oral syringe and give right away. If your child does not like the taste of the liquid, ask your child’s health-care provider about strategies to help with giving the medication. 

What should you do if your child misses a dose of mexiletine?

• Give the missed as soon as you remember.
• If it is almost time for the next dose, skip the missed dose. Give the next dose at the regular time.
• Do not give two doses at once to make up for one missed dose.

What are the possible side effects of mexiletine?

Mexiletine may cause side effects. Tell your child’s health-care provider if any of these symptoms are severe or do not go away:

• nausea
• vomiting
• heartburn
• changes in appetite
• light-headedness or dizziness
• shaking of a part of your child's body that they cannot control
• loss of coordination
• numbness or tingling sensation
• headache
• blurred vision
• difficulty speaking
• swelling of the hands, feet, ankles or lower legs
• rash
• feeling anxious or nervous
• feeling tired

Some side effects can be serious. If your child experiences any of these symptoms, call their health-care provider immediately:

• irregular heartbeat
• chest pain
• extreme tiredness
• unusual bleeding or bruising
• lack of energy
• pain in the upper-right part of the stomach
• yellowing of the skin or eyes
• flu-like symptoms
• changes in balance (i.e., tripping or falling often)
• significant reduction in amount of urine (peeing much less than usual)

Mexiletine may cause other side effects. Call your doctor if you have any unusual problems while taking this medication.

If your child has collapsed, had a seizure, has trouble breathing or can't be awakened, immediately call 911.

What safety measures should you take when your child is using mexiletine?

Do not give grapefruit or grapefruit-juice containing foods and drinks when your child is taking mexiletine. This can increase mexiletine levels in the blood and cause harmful side effects.

Because of mexiletine’s effects on heart rhythm, your child’s health-care provider may sometimes check your child’s heart rhythm using a test called an ECG (electrocardiogram) to ensure everything is normal. This test is not painful and is only slightly uncomfortable. An ECG can be done in a clinic.

There are some medicines that should not be taken together with mexiletine, or, in some cases, the dose of mexiletine or the other medicine may need to be adjusted. It is important that you tell your child’s health-care provider if your child takes any other medications (prescription, over the counter or herbal), including:

• clozapine
• duloxetine
• a seizure medication called phenytoin
• an antibiotic for treating tuberculosis called rifampin
• heart medications that can shorten the qt interval, including propranolol and nadolol

Other important information about mexiletine

• Keep this medication in the container it came in, tightly closed and out of reach of children. Store it at room temperature and away from excess heat and moisture (not in the bathroom).
• Keep a list of all medications your child is on and show the list to their health-care provider or pharmacist.
• Do not keep any medicines that are out of date. Check with your pharmacist about the best way to throw away outdated or leftover medicines.

Do I need to follow up after starting mexiletine?

It is important to follow up with your child’s neurologist after starting mexiletine to ensure it is working properly and to check if any dose adjustments are needed. During these visits, make sure to mention any discomfort or side effects they may be experiencing. Regular check-ins with your child’s health-care team will help make sure the medication is providing the best benefit.

References

Lexicomp. (2026). Mexiletine.  

MedlinePlus. (2016). Mexiletine. https://medlineplus.gov/druginfo/meds/a607064.html

At SickKids

SickKids has a specialized neuromuscular clinic that focuses on myotonia and similar conditions. Located within the neurology department, the clinic is staffed by a neurologist, nurse practitioner and experienced technicians who are dedicated to caring for children with neurological disorders. Our clinic visits range from simple follow-ups to more specialized procedures, such as nerve conduction studies and electromyography. Additionally, we offer access to a professional social worker who can provide support for both you and your child and connect you with relevant support groups if needed.

To learn more about the Neuromuscular Program at SickKids, visit https://www.sickkids.ca/en/care-services/clinical-departments/neurology/neuromuscular-program/.

Keep mexiletine out of your child’s sight and reach and locked up in a safe place. If your child takes too much mexiletine, call your local Poison Information Centre at one of these numbers. These calls are free. 

• Call 1-844 POISON-X, or 1-844-764-7669, from anywhere in Canada except Quebec.
• Call 1-800-463-5060 if you live in Quebec.