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Methimazole for Cats

By | Last Updated: 22nd February 2023

What is methimazole used for in cats

Methimazole (the active ingredient in Tapazole) is an effective veterinary medication used for the management of hyperthyroidism in pets, a condition that causes an excessive production of thyroid hormone in the body [1, 2]. While there are two drugs available, methimazole and propylthiouracil, for medical therapy of feline hyperthyroidism, the former is more commonly used because it has a lower rate of adverse side effects compared to the latter [3].

Aside from its use for lowering the thyroid hormone concentration in blood, the medicine may also be administered before a radioactive iodine therapy or surgery of the thyroid gland [5].

Quick Information

Methimazole for Cats

Generic name: Methimazole

Brand name: Tapazole® [2]

Type: Thioureylene anti-thyroid agent [1]

Does FDA approve it: Yes [1]

Availability: Prescription medication [2]

Who can take it: Cats, dogs, humans [2]

Forms: 5 mg and 10 mg white round tablets, transdermal gel, cream, or liquid [1, 2]

Cost: $0.46 – $0.47 per 5mg tablet

Mechanism of action: how does it work

The production of thyroid hormones, including triiodothyronine (T3) and thyroxine (T4), is initiated by the peroxidase enzyme through the oxidation reaction of tyrosyl groups of thyroglobulin protein and iodide ion. Methimazole interferes with the regular interaction of peroxidase and iodine with thyroglobulin and reduces the formation of thyroid hormones [6, 10].

Methimazole dosage for cats

Talk to a vet to determine your cat’s daily dosage requirements and the best time to administer.

The usually recommended initial dose for cats of all weight is 2.5 mg, given twice at an interval of 8 to 12 hours per day [11]. The maximal daily dosage varies depending on the severity of the condition, from 2.5 mg for a mild case to 5 mg for a moderate case, and 10-15 mg for a severe chronic condition [11].

Blood tests are usually repeated every 28 days until the hormone level reaches a steady state, and the maintenance dosage is achieved [7].

How to give methimazole to your cat

Give the medication as instructed by the vet. As a tablet, it can be administered with or without food. However, you need to check with the vet before dividing the tablet or crushing it to mix with your cat’s food [12]. Make sure you give your pet plenty of water to drink [8].

It may also be provided as an oral liquid [4]. Since methimazole tastes bitter, you may choose the flavored varieties that are compounded in some pharmacies [22].

How to apply topical methimazole to cats

For the transdermal cream or gel, wear gloves and measure the dose onto your finger before gently applying it on your cat’s skin (usually to the pinna) [4]. Clean the site using a small piece of moist cloth before giving the next dose.

Do not stop applying the cream unless the vet advises you to do so [4].

What to do if you miss giving a dose

If you have missed a scheduled dose, give it as soon as you can. However, it is best to skip the dose if the next dose is due within a few hours. Make sure never to provide a double dose [4].

Symptoms of an overdose

  • Easy bruising and bleeding
  • Headache (your cat pushing its head against the floor or other objects, hiding its face in its paws, becoming sluggish, sleeping longer, and/or being hyper-reactive to touch specifically on its head or neck) [13]
  • Fever (watch for signs like decreased drinking, loss of appetite, shivering, rapid breathing, and lack of activity) [14]
  • Itching or increased scratching
  • Swelling
  • Nausea
  • Upset stomach (look for signs like loss of appetite, diarrhea, or vomiting) [9, 15]

If your cat shows any of the abovementioned signs of overdose, urgently call your vet or the ASPCA Animal Poison Center at 1-888-426-4435.

How long does it take for methimazole to work

Although studies have revealed that methimazole can effectively work for 2.3 hours in hyperthyroid cats and 4.7 hours in normal cats, a single dose can actually suppress thyroid hormone levels for about 24 hours [10, 11, 20].

Methimazole (Tapazole) side effects

The following common side effects may occur within the first one to three months of treatment:

  • Vomiting
  • Headache
  • Dizziness
  • Drowsiness
  • Hair loss
  • Joint, nerve, or muscle pain (difficulty jumping, reluctance to move, licking a specific body part, or abnormal gait) [19]

Other serious side effects may include:

  • Difficulty breathing (the cat opening its mouth, sticking its elbows out from its body, flaring up its nostrils, extending its head and neck out in front while breathing) [17]
  • Fever
  • Swelling of the face, lips, tongue
  • Loss of appetite
  • Bloody stool or urine
  • Vomiting
  • Depression (losing appetite, becoming less active, displaying abnormal behavior, or disturbed sleep patterns) [18]
  • Autoimmune disease like jaundice [8]

Contact your veterinarian if your cat has any of the above side effects.

When should it not be given to cats

Make sure to discuss with your vet of any existing health conditions in your cat. Methimazole is not prescribed for cats that have:

  • Anemia
  • Kidney disorders
  • Liver disease
  • Lymphopenia
  • Neutropenia
  • Thrombocytopenia

Its use is also not recommended for pregnant and nursing cats because it may be toxic to an embryo and cause birth defects [1].

Potential drug interactions

Be sure to inform the vet if your cat is on any of the following medications:

  • Warfarin (Coumadin)
  • Beta blockers like atenolol (Tenormin)
  • Propranolol (Inderal)
  • Metoprolol (Lopressor)
  • Digoxin (Lanoxin)
  • Theophylline (Theo-Dur) [8, 9]

Homeopathic and herbal remedies, as well as acupuncture, are considered alternatives for a methimazole treatment. However, there is not enough evidence to prove their effectiveness [21].


    1. Focus On Pharmacology: Methimazole: Management of Feline Hyperthyroidism – Todaysveterinarypractice.navc.com
    2. Methimazole – Petmd.com
    3. Clinical efficacy and safety of transdermal methimazole in the treatment of feline hyperthyroidism – Ncbi.nlm.nih.gov
    4. Methimazole – Vcahospitals.com
    5. Methimazole – 1800petmeds.com
    6. Methimazole, Tapazole – Medicinenet.com
    7. About Tapazole Tablets for Hyperthyroidism in Cats – Petcarerx.com
    8. Tapazole – 1800petmeds.com
    9. Tapazole for Cats – Vetinfo.com
    10. How to Dose and Monitor Hyperthyroid Cats on Methimazole – Animalendocrine.blogspot.in
    11. Methimazole (Veterinary – Systemic) – Aavpt.org
    12. Getting Your Cat to Take Medication – Thecatvet.co.uk
    13. Cat Headaches – Cats.lovetoknow.com
    14. Fevers in Cats – Pets.webmd.com
    15. Remedy for an Upset Stomach in a Cat – Pets.thenest.com
    16. Methimazole for Cats – Wedgewoodpetrx.com
    17. Breathing Difficulties in Cats – Petmd.com
    18. Can Cats Get Depressed? – Petmd.com
    19. 25 Red Flags That Spell Pain – Healthypets.mercola.com
    20. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats – Ncbi.nlm.nih.gov
    21. Feline Hyperthyroidism – Healthypets.mercola.com
    22. Methimazole for Cats and Vomiting – Pets.thenest.com

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