By Rania Gollakner, BS, DVM, MPH

What is mexiletine?

Mexiletine (brand names: Mexitil®, Mexilen®, Mexitilen®, Myovek®, Ritalmex®) is an antiarrhythmic drug used to treat abnormal heart rhythms, specifically certain ventricular arrhythmias such as premature complexes and ventricular tachycardia in dogs. It has also been used to treat muscle disorders in certain breeds of dog.

Its use in dogs to treat abnormal heart rhythms is ‘off label’ or ‘extra label’. Many drugs are commonly prescribed for off label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions may be significantly different from those on the label.

How is mexiletine given?

Mexiletine is given by mouth in the form of a capsule. It may also be compounded into a liquid form. It can be given with or without food, however it should be given with food to reduce the risk of vomiting and nausea. Measure liquid forms carefully. This medication should take effect within 1 to 2 hours; however, effects may not be visibly obvious and therefore laboratory tests may need to be done to evaluate this medication’s effectiveness.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember, but if it is close to the time for the next dose, skip the dose you missed and give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses.

Are there any potential side effects?

The most common side effects include vomiting, nausea, diarrhea, constipation, trembling, dizziness, clumsiness, or anxiety. More serious side effects include difficulty breathing or shortness of breath, abnormal heart rhythms, seizures, and low blood cell counts that could cause bleeding or infections.

This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

Mexiletine should not be used in pets that are allergic to it. It should not be used (unless absolutely necessary) in pets with certain abnormal heart rhythms, cardiogenic shock, severe congestive heart failure, impaired liver or kidney function, low blood pressure, or seizures. Mexiletine should be used cautiously in pregnant pets. If this medication is used in a lactating pet, use a milk replacer.

Some breeds of dogs (e.g., Collies, Sheepdogs, and Collie- or Sheepdog-cross breeds) are more sensitive than others to certain medications. This is typically due to a specific genetic mutation (MDR1) that makes them less able to tolerate high doses of certain medications. Therefore, use mexiletine extremely cautiously in these cases.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with mexiletine: amiodarone, antacids, atropine, bupropion, cimetidine, clozapine, griseofulvin, lidocaine, metoclopramide, opioids, paroxetine, phenobarbital, primidone, phenytoin, propranolol, quinidine, rifampin, sotalol, theophylline/aminophylline, urinary acidifying drugs, or urinary alkalinizing drugs.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

Your veterinarian will monitor your pet’s heart rhythms with periodic electrocardiograms (ECGs) to be sure that the medication is working. Monitor your pet for serious side effects.

How do I store mexiletine?

Mexiletine should be stored at room temperature and in a tight container protected from moisture.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility.

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