Methimazole
(Felimazole, Tapazole)

Common Drug Name: Methimazole

Common Brand Names: Tapazole (human form) Felimazole (veterinary form) Generic products are available.

Storage: Refrigerate oral suspension. Store other forms at room temperature in a tightly closed container protected from light.

Uses: Cats: Methimazole reduces the amount of thyroid hormone produced by the body. It is used to treat hyperthyroidism, a condition in which too much thyroid hormone is produced. It will not cure the disease, but will usually control it if given for the rest of the cat’s life. Dogs: Methimazole may be used to protect the kidneys in dogs receiving cisplatin, an anti-cancer drug.

Dose and Administration: Always follow the dosage instructions provided by your veterinarian. If you have difficulty giving the medication, contact your veterinarian. Methimazole in tablet form is very bitter, and it may be difficult to administer. Compounding pharmacies can make a flavored oral liquid form or transdermal (topical) gel, which may be easier to give. If using the transdermal gel, apply to the skin as directed by your veterinarian. Blood levels of thyroid hormone will be checked by your veterinarian at regular intervals, and the dose increased or decreased as needed. The lowest effective dose should be used. If you miss a dose, give it as soon as you remember. If it is almost time for the next dose, skip the one you missed and go back to the regular schedule. Do not give 2 doses at once. This medication should only be given to the pet for whom it was prescribed.

Possible Side Effects: Side effects tend to occur in the first three months of treatment. If you observe any of the effects listed below, contact your veterinarian immediately. May see loss of appetite, vomiting, lethargy or abnormal vocalization. These usually occur within the first two weeks of treatment and may stop even with continuation of treatment. Less common side effects include liver problems resulting in the above signs plus yellowing of the gums, skin, or eyes; skin lesions or itching of the face, resulting in scratching; bleeding tendencies; swollen lymph nodes; anemia; or low white blood cell counts. Your veterinarian will determine if these effects necessitate stopping the medication and treating with surgery or radioactive iodine therapy. Treatment with methimazole may unmask hidden kidney disease. If your pet is unusually tired, has a fever (temperature over 103°F), or shows signs of bruising or bleeding, contact your veterinarian immediately. If your pet experiences an allergic reaction to the medication, signs may include facial swelling, hives, scratching, sudden onset of diarrhea, vomiting, shock, seizures, pale gums, cold limbs, or coma. If you observe any of these signs, contact your veterinarian immediately.

Precautions: Not for use in pets who are hypersensitive (allergic) to it. Use with extreme caution in pets with anemia, clotting disorders, bleeding, low white cell and platelet counts, or diseases of the immune system, liver, or kidney. May cause birth defects. Do not use in pregnant or lactating animals (females nursing their young). In addition to monitoring of thyroid hormone levels, laboratory tests to check liver and kidney function, and blood cell counts may be performed before starting treatment and then regularly thereafter.

Human Precautions: NOTE: Methimazole can cause birth defects. Pregnant women should be extremely cautious about handling this medication or waste products from a cat receiving this medication. All people should wear gloves when applying the gel form or giving the pill form and wash your hands well after use. Do not break or crush tablets. Wear protective gloves to prevent direct contact with litter, feces, urine or vomit of treated cats, and broken or moistened tablets. Wash hands after contact with the litter of treated cats. Persons with low thyroid function should use extreme care when handling the drug and avoid all skin contact with it.

 Drug, Food and Test Interactions: Consult your veterinarian before using methimazole with any other medications, including vitamins and supplements, since interactions may occur. The drugs clomipramine, amitriptylline, omeprazole, and cyclophosamide may interact with methimazole and cause severe side effects.

Signs of Toxicity/Overdose: Signs of toxicity or overdose are similar to the side effects listed; liver and blood disorders are the most common. If a known or suspected overdose has occurred, or if you observe any of these signs in your pet, contact your veterinarian immediately. Keep this and all other medications out of the reach of children and pets.

** This information may not cover all possible uses, directions, side effects, precautions, allergic reactions, drug interactions, or withdrawal times.  Always consult your own veterinarian for specific advice concerning the treatment of your pet.

If you have questions about this medication, please call our Pharmacy at (877) 815-6337 for a consultation