Methimazole is the most commonly prescribed medication for hyperthyroidism in cats, a condition caused by an overactive thyroid gland. Hyperthyroidism is one of the most frequently diagnosed endocrine disorders in middle-aged and older cats, and methimazole remains the cornerstone of medical management for this condition.
Available under the brand names Tapazole (human formulation) and Felimazole (FDA-approved veterinary formulation), methimazole is now also available as a generic, with a first generic version approved in July 2025, potentially improving affordability for cat owners managing this lifelong condition.
What Is Methimazole Used For?
Methimazole is used to treat feline hyperthyroidism, a condition in which the thyroid gland produces excessive amounts of thyroid hormones (T3 and T4). This overproduction affects nearly every organ system and can cause:
- Weight loss despite an increased appetite
- Increased thirst and urination
- Vomiting and diarrhea
- Hyperactivity and restlessness
- Rapid heart rate and heart murmur
- Poor coat condition
- High blood pressure
Methimazole does not cure hyperthyroidism. It controls thyroid hormone levels for as long as the medication is given. If methimazole is stopped, thyroid hormone levels will rise again. For this reason, methimazole treatment is typically lifelong unless the cat undergoes a curative treatment such as radioactive iodine therapy (I-131) or surgery.
Methimazole may also be used as a short-term trial treatment before radioactive iodine or surgery to stabilize thyroid levels and assess kidney function.
How Does Methimazole Work?
Methimazole works by blocking the enzyme thyroid peroxidase, which is essential for the synthesis of thyroid hormones. By inhibiting this enzyme, methimazole reduces the production of T3 and T4, bringing hormone levels back toward the normal range.
Methimazole does not shrink or destroy thyroid tissue. It only suppresses hormone production while the drug is present in the body. This is why consistent, daily administration is necessary.
Dosage
Your veterinarian will determine the exact dose for your cat. The following are typical starting ranges reported in veterinary literature.
| Formulation | Typical Starting Dose | Frequency | Notes |
|---|---|---|---|
| Oral tablets (Felimazole, Tapazole, generic) | 1.25-2.5 mg per cat | Every 12 hours (twice daily) | Dose adjusted based on blood work |
| Transdermal gel (compounded) | 2.5 mg per cat | Every 12 hours (applied to inner ear pinna) | For cats that resist oral medication |
Important notes:
- Your veterinarian will check thyroid levels (T4) and kidney values approximately 2-3 weeks after starting methimazole, and will adjust the dose based on results.
- Most cats require dose adjustments over time. Some cats eventually need higher doses as the condition progresses.
- The goal is to bring T4 levels into the normal range while monitoring kidney function closely.
- Never adjust the dose without consulting your veterinarian.
How to Give Methimazole to Your Cat
Methimazole is available in three main forms:
Oral Tablets
- Felimazole is FDA-approved for cats and comes in 2.5 mg and 5 mg sugar-coated tablets.
- Tapazole and generic methimazole tablets are the human formulation and are commonly used off-label in cats.
- Tablets can be given directly, hidden in a pill pocket, or crushed and mixed into a small amount of food (consult your vet, as crushing coated tablets may affect absorption).
Transdermal Gel
- A compounding pharmacy can prepare methimazole as a gel that is applied to the inner surface of the ear pinna (ear flap).
- This option is popular for cats that are difficult to pill.
- Wear gloves when applying the gel, as methimazole can be absorbed through human skin.
- Alternate ears with each application to reduce the risk of local irritation.
- Absorption from transdermal application may be less consistent than oral dosing; your veterinarian will monitor blood levels accordingly.
Compounded Liquid
- Compounding pharmacies can also prepare flavored liquid formulations.
- These may be helpful for cats that need precise dose adjustments in small increments.
Human safety precaution: Methimazole can be absorbed through the skin and may pose a risk to people, particularly pregnant women or those with thyroid conditions. Always wear disposable gloves when handling the transdermal gel, and wash your hands after handling any methimazole formulation.
Side Effects
Most cats tolerate methimazole well, especially at lower starting doses. Side effects are more common in the first few weeks of treatment and may resolve on their own. Contact your veterinarian if side effects persist or worsen.
| Common Side Effects (first 1-3 months) | Serious Side Effects (Contact Your Vet Promptly) |
|---|---|
| Decreased appetite | Facial itching or excoriation (scratching at the face and neck) |
| Vomiting | Jaundice (yellowing of the skin, gums, or ears) |
| Lethargy | Bleeding or bruising (could indicate bone marrow suppression) |
| Mild GI upset | Severe or persistent vomiting |
| Collapse or extreme weakness |
Bone marrow suppression is a rare but serious side effect of methimazole. This can result in low white blood cell counts (making the cat susceptible to infections), low platelet counts (increasing bleeding risk), or anemia. Regular blood work monitoring is essential, especially during the first 3 months of treatment.
Unmasking kidney disease: One of the most important considerations with methimazole treatment is the relationship between hyperthyroidism and kidney function. Hyperthyroidism increases blood flow to the kidneys, which can mask underlying chronic kidney disease (CKD). When thyroid levels are normalized with methimazole, kidney function may decline. Your veterinarian will monitor kidney values carefully, especially during the first weeks of treatment, and may need to adjust the methimazole dose to balance thyroid control with kidney function.
Drug Interactions
- Hepatotoxic drugs: Methimazole may increase the risk of liver damage when used with other drugs that affect the liver.
- Bone marrow-suppressive drugs (chlorambucil, some antibiotics): Increased risk of blood cell abnormalities.
- Iodinated contrast agents: May interfere with thyroid testing during treatment.
Inform your veterinarian about all medications and supplements your cat is receiving.
Contraindications
Methimazole may not be suitable for cats with:
- Known hypersensitivity to methimazole or related compounds
- Pre-existing liver disease
- Pre-existing blood disorders (autoimmune hemolytic anemia, thrombocytopenia)
Your veterinarian will weigh the risks and benefits based on your cat’s overall health.
Storage
- Store oral tablets at room temperature (59-77 degrees F / 15-25 degrees C), protected from light and moisture.
- Transdermal gel: Follow the compounding pharmacy’s storage instructions (typically refrigerated).
- Keep all formulations out of reach of children and other pets.
Cost & Availability
Methimazole is available as a brand-name veterinary product (Felimazole), the human brand (Tapazole), and as a generic.
- Felimazole (2.5 mg, 100 tablets): Approximately $50-$80.
- Generic methimazole tablets: Approximately $10-$30 for a 30-day supply, depending on the dose and pharmacy. The first generic version approved in July 2025 may further improve affordability.
- Transdermal gel (compounded): Approximately $30-$60 for a 30-day supply, depending on the compounding pharmacy.
- Monthly monitoring blood work: Typically $50-$150 per test, more frequent in the first few months.
Methimazole tablets (generic and Tapazole) can often be filled at human pharmacies with a veterinary prescription, which may offer savings. Felimazole and transdermal gel are typically obtained through veterinary clinics or veterinary pharmacies.
Because methimazole treatment is lifelong for most cats, the cumulative cost of medication and monitoring is an important consideration. Discuss all treatment options (including radioactive iodine as a potential one-time cure) with your veterinarian.
FAQ
Q: Is methimazole a cure for hyperthyroidism? A: No. Methimazole controls thyroid hormone production but does not eliminate the underlying thyroid abnormality. If the medication is stopped, hormone levels will rise again. Radioactive iodine therapy (I-131) is the only widely available curative option. Consult your veterinarian about whether your cat is a candidate.
Q: How often does my cat need blood work while on methimazole? A: Your veterinarian will typically check blood work (thyroid levels and kidney values) 2-3 weeks after starting treatment or after any dose change. Once your cat is stable, monitoring is usually recommended every 3-6 months. More frequent testing may be needed for cats with concurrent conditions.
Q: Can the transdermal gel be used long-term? A: Yes, many cats use the transdermal gel as their primary methimazole formulation for years. However, your veterinarian may periodically reassess whether the gel is achieving adequate thyroid control, as transdermal absorption can vary between cats.
Q: Will treating hyperthyroidism cause kidney problems? A: Treating hyperthyroidism does not cause kidney disease, but it can unmask pre-existing kidney disease that was previously hidden by the elevated blood flow associated with hyperthyroidism. This is why kidney function monitoring is a critical part of methimazole therapy. Your veterinarian may start with a low dose and increase gradually to assess the impact on kidney values.
Q: Is the new generic methimazole the same as Felimazole? A: Generic methimazole contains the same active ingredient as Felimazole. The FDA requires generics to meet bioequivalence standards. However, inactive ingredients (coatings, fillers) may differ. If your cat has been stable on one formulation, consult your veterinarian before switching to confirm the dose remains appropriate.
Sources & References
- FDA - Felimazole (Methimazole) Approval Information
- Merck Veterinary Manual - Hyperthyroidism in Cats
- VCA Animal Hospitals - Methimazole for Cats
- Cornell Feline Health Center - Hyperthyroidism in Cats
- American Association of Feline Practitioners - Feline Thyroid Guidelines
This content is for informational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian before starting, changing, or stopping any medication for your cat.