Methimazole for Cats (Felanorm, Felimazole, Tapazole)

PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function,a their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.

What Is Methimazole Used for in Cats?

Methimazole is a prescription anti-thyroid medication used for the management of hyperthyroidism in cats. 

Hyperthyroidism is a condition that occurs when there is excess thyroid hormone in a pet’s system. The thyroid hormone helps regulate a cat’s metabolism. Symptoms of hyperthyroidism can include:

• Weight loss

• Increased vocalization

• Vomiting

• Increased drinking

• Increased urination

Senior cats are more commonly diagnosed with the disease.

Methimazole Brand Names

Methimazole is available under brand names Felanorm, Felimazole, and Tapazole in a variety of formulations.

• Felanorm is the first FDA-approved generic methimazole oral solution for hyperthyroidism in cats. Formulated as a honey-flavored solution, Felanorm is readily accepted and convenient to administer. The oral solution formula allows for more accurate dosing than pills.

• Felimazole is the first FDA-approved methimazole tablet for use in cats.

• Tapazole, on the other hand, is FDA-approved for humans and was used extra-label in cats prior to the FDA approval of Felimazole in 2009 and Felanorm in 2024.

  • Tapazole is currently not FDA-approved as a veterinary medication, but it is readily utilized in the veterinary field, and veterinarians can legally prescribe certain human drugs for animals in certain circumstances.

  • This is called extra-label or off-label use because this use isn’t described on the drug label. Your veterinarian will determine whether this medication is right for your pet.

Compounded Methimazole 

In certain circumstances, your vet may recommend a compounded formulation of methimazole.

Compounded medications are prescribed if there’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if your pet has trouble taking pills in capsule form, the dosage strength is not commercially available, or the pet is allergic to an ingredient in the FDA-approved medication.

Compounded medications are not FDA-approved. They’re created by either a veterinarian or a licensed pharmacist on an individual basis to best suit a patient’s particular needs.

How Methimazole Works in Cats

Methimazole, the active ingredient in Felimazole tablets and Felanorm liquid, works by blocking the excess production of thyroid hormones from an overactive thyroid gland.

Methimazole is not a cure for hyperthyroidism because the underlying cause of the overactive thyroid gland is still present. Methimazole is given on a strict schedule to prevent the formation of excessive levels of thyroid hormone that cause damage to body systems.

Your cat will need to be given this medication consistently for the prevention of symptoms associated with hyperthyroidism. 

Methimazole for Cats Considerations

Methimazole should not be given to:

• Cats with liver disease, autoimmune diseases, kidney disease, or other blood abnormalities

• Pregnant or lactating cats

• Cats that are allergic to methimazole

• Cats allergic to carbimazole or polyethylene glycol 

Giving methimazole with certain medications can result in health risks to your cat, so it’s important to discuss your cat’s medications, including vitamins and supplements, and medical conditions with your veterinarian.

Methimazole for Cats Directions

Follow the directions on the drug label or as provided by your veterinarian.

Dosing is highly dependent on an individual cat’s needs. 

Missed a Dose? 

Speak with your veterinarian about what to do if you forget to give a dose of methimazole.

Generally, your vet may instruct you to give it when you remember, or if it’s almost time for your pet’s next dose, to skip the missed dose and resume your normal dosing schedule.

Do not give extra or double doses. 

Possible Side Effects of Methimazole in Cats

Side effects from methimazole for hyperthyroidism typically occur within the first three months of use.

The most common side effects are: 

• Not eating (anorexia)  

• Vomiting 

• Diarrhea

• Depression

• Mild to moderate changes in your pet’s white blood cell count, which typically self-resolve  

Rare side effects can include: 

• Severe facial itching  

• A decrease in blood platelet levels 

• Liver disease

• Immune-mediated disorders 

• Lowered levels of thyroid hormone in the body can also uncover underlying kidney disease if present 

Call your veterinarian if you see any of these side effects in your cat.

Human Side Effects 

Felimazole and Felanorm (both containing the active ingredient methimazole) and compounded methimazole for cats are prescription veterinary medications not intended for use in humans.

Due to possible side effects, humans should never use medicine dispensed for their pets and pets should not be given any medicine dispensed for a human’s use.

It’s important to keep in mind that methimazole can also affect the human thyroid and to minimize your exposure when administering it to your cat.

Here are some tips for handling methimazole:

• Do not break or crush Felimazole tablets.

• Avoid skin and oral exposure with Felanorm oral solution. Wash any spillages or splatter from the skin immediately.

• Wear gloves during administration of the medication and wash your hands with soap and water afterwards to minimize any potential exposure to the drug.

• If you’re pregnant or nursing, wear gloves when handling this medication or when exposed to the bodily fluids of cats (litter, feces, urine, or vomit) being given methimazole. 

In case of accidental eye contact, rinse eyes immediately with clean running water. If irritation develops, seek medical advice.

If you accidentally ingest this medication, seek medical advice immediately and call the national Poison Control Center hotline at 800-222-1222.

Monitoring Your Pet

Cats treated with methimazole for hyperthyroidism should have their thyroid levels and overall blood and urine checked before starting treatment.

Once treatment has started, a pet’s thyroid levels should be monitored after three weeks and again after six weeks of treatment. Once appropriate testing levels are met, thyroid levels should be monitored every three months, and doses can be adjusted as needed.

Additional testing may be recommended by your veterinarian. 

When To Call Your Vet

Call your veterinarian if any of the following apply:

• Severe side effects are seen (see above) 

• You see or suspect an overdose

• Your pet’s condition worsens or does not improve with treatment

• You have additional questions or concerns about the use of methimazole 

Methimazole Overdose in Cats Information

One-time overdoses may cause the adverse reactions listed above.

White blood cell changes, liver disease, low platelet levels, and other serious side effects can also be seen.

If you suspect an overdose, immediately contact your veterinarian or an animal poison control center. Consultation fees often apply. 

• Pet Poison Helpline 855-764-7661 

• ASPCA Animal Poison Control 888-426-4435 

How To Store Methimazole for Cats

Here are some tips for storing methimazole:

• Compounded medications should be stored according to the compounding pharmacy’s label. 

• Methimazole should be stored at controlled room temperatures of 68–77 F (20–25 C). Brief exposure to temperatures of 59–86 F (15–30 C) are acceptable.

• Keep the container tightly closed to protect from moisture and light. 

• Keep out of reach of children and pets.