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Methimazole by Sandoz

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Methimazole is used to treat hyperthyroidism in cats. It has largely replaced propylthiouracil in this treatment process since it has a lower incidence of adverse side effects.



  • Effectively treats hyperthyroidism in cats
  • Rapid results
  • Affordably sold per tablet

How it works:
Hyperthyroidism is a condition that occurs when the thyroid gland produces too much thyroid hormone. Methimazole inhibits the production of thyroid hormones.

Blood tests must be done to check for proper dosage. Do not use in pregnant or nursing animals. Do not give your pet a live vaccine while giving methimazole. Contact your veterinarian immediately if your pet develops signs of an infection.

What is the most important information I should know about Methimazole:
Methimazole is a prescription medication not FDA approved for veterinary use; however, it is a commonly accepted practice for veterinarians to use this medication in cats. Methimazole is available as a 5mg scored tablet. The usual initial dose for cats is 5mg every 8 to 12 hours. Do not give methimazole to pregnant or nursing animals. Methimazole can increase the risk of bleeding. Methimazole can lower the blood cells that help fight infections. Your veterinarian will need to give blood tests on a regular basis to be sure these blood cells do not get too low. Do not give the pet a "live" vaccine while the pet is taking methimazole. Methimazole is needed for the life of the pet. If the medication is stopped, the symptoms will reappear.

What is Methimazole:
Methimazole prevents the thyroid gland from producing too much thyroid hormone. Methimazole is used to treat overactive thyroid (hyperthyroidism). It is also used before thyroid surgery or radioactive iodine treatment. Methimazole may also be used for purposes other than those listed in this guide.

What should I discuss with my veterinarian before giving Methimazole to my pet:
Tell your veterinarian if your pet has an allergy to methimazole or if your pet has liver disease, a blood cell disorder, or a weak immune system. Tell your veterinarian if the pet is pregnant or nursing. Inform your veterinarian of any other medications, including vitamins and supplements your pet may be taking while receiving methimazole.

How should this medication be given:
Give this medication exactly as directed by your veterinarian. Do not give in larger amounts, or give it for longer than recommended by your veterinarian. Follow the directions on the prescription label. If you do not understand the directions ask the pharmacist or veterinarian to explain them to you. Methimazole can be given with or without food. Allow plenty of water for the pet to drink. Store methimazole at room temperature away from moisture and heat. Keep this medication away from children and pets.

What happens if I miss giving a dose:
Give the missed dose as soon as you remember during the same day. However, if you don't remember until the next day, skip the dose you missed and give only the next regularly scheduled dose. Do not give a double dose of the medication.

What happens if I overdose the pet:
Seek emergency veterinary medical treatment. Symptoms of methimazole overdose may include nausea, vomiting, upset stomach, joint pain, headache, fever, itching, swelling, and easy bruising or bleeding.

What should I avoid while giving Methimazole to my pet:
Do not give the pet a "live" vaccine while the pet is taking methimazole. Contact your veterinarian at once if your pet develops signs of an infection.

What are the possible side effects of Methimazole:
If any of the following serious side effects occur, stop giving methimazole and seek emergency veterinary medical attention; an allergic reaction (difficulty breathing; swelling of the lips; tongue or face; hives). Stop giving methimazole and contact your veterinarian at once if your pet develops any of these serious side effects; fever, chills, body aches; easy bruising or bleeding, unusual weakness; blood in the urine or stools; severe blistering, peeling, and skin rash; nausea, low fever, loss of appetite, dark urine, jaundice. Other less serious side effects may occur. Continue giving methimazole and talk to your veterinarian if your pet experiences headache, drowsiness, dizziness; mild nausea, or vomiting; itching; muscle, joint, or nerve pain; swelling; hair loss. Other side effects may also occur. Talk to your veterinarian about any side effect that seems unusual or bothersome to the animal.

What other drugs will affect Methimazole:
Tell your veterinarian if your pet is taking theophylline (Theo-Dur), warfarin (Coumadin), digoxin (Lanoxin), a beta blocker such as atenolol (Tenormin), metoprolol (Lopressor), or propranolol (Inderal). Drugs other than those listed may also interact with Methimazole. Talk to your veterinarian or pharmacist before giving any prescription or over the counter medicines.


(Felimazole, Tapazole) 

Common Drug Name


Common Brand Names
Tapazole (human form)
Felimazole (veterinary form)

Generic products are available.

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

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. Do not split or crush enteric-coated tablets.

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.

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 or nursing women should be extremely cautious about handling this medication or handling waste products from a cat receiving this medication. All people should wear protective gloves when splitting tablets, applying the gel form or giving the pill form, and wash hands well after use. Do not split or crush enteric-coated 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.

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