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Felimazole by Dechra Veterinary Products

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Felimazole™ Coated Tablets

Generic Name:


General Description:

Methimazole is an inexpensive drug used in cats to treat an overactive thyroid gland (hyperthyroidism). Your cat's abnormal appetite, weight loss and heart issues will go back to normal once the thyroid hormone levels are restored. It is also given as a kidney protectant to dogs receiving cisplatin chemotherapy. It is available in 5 mg scored tablets.

What is this drug?
  • A drug which blocks the production of thyroid hormones
  • Given by mouth
Reasons for prescribing:
  • To control hyperthyroidism in the cat
  • To protect the kidneys in dogs receiving cisplatin chemotherapy
What dogs/cats should not take this medication?
  • Use with extreme caution in pets with anemia, clotting disorders, bleeding, low white cell and platelet counts, kidney, liver or immune system diseases
  • Use with extreme caution in pregnant or nursing pets. The young should be placed on milk replacer after they've nursed colostrum (first milk immediately after birth)
  • If your pet has had an allergic reaction to methimazole or like products

Read and follow the label carefully. Ideally, give the medication at the same time(s) daily.

Give the exact amount prescribed and only as often as directed. It is usually given one to three times a day . If your cat will not take tablets, methimazole can be formulated as a flavored liquid to be mixed in the food, or it can be compounded as a gel to be applied on the hairless skin of the inside of your cat's ear.

Cats: As this drug is not a cure, but is used to control hyperthyroidism, it is usually given for the remainder of the cat's life.

Before starting therapy, baseline blood work should be performed to assess thyroid levels and your pet's overall health. Periodic blood work will be necessary to monitor thyroid levels and the drug's effect on your pet's health. Dose adjustments will be made based upon these results and an assessment of how your cat is responding clinically.

Transdermal gel: Wear gloves and wash your hands after handling.

Call ahead for refills.

What if a dose is missed?

If a dose is missed, give it as soon as you can. If it is time already for the next dose, skip the missed dose and go back to the normal schedule. Do not give two doses at the same time.

What to tell/ask veterinarian before giving medication?

Talk to your veterinarian about:

  • When will your pet need to be rechecked
  • What tests may need to be performed prior to and during treatment with this drug
  • What are the risks and benefits of using this drug

Tell your veterinarian about:

  • If your pet has experienced side-effects on other drugs/products
  • If your pet has experienced digestive upset now or ever
  • If your pet has experienced liver or kidney disease now or ever
  • If your pet has experienced any other medical problems or allergies now or ever
  • All medicines and supplements that you are giving your pet or plan to give your pet, including those you can get without a prescription. Your veterinarian may want to check that all of your pet's medicines can be given together.
  • If your pet is pregnant or nursing or if you plan to breed your pet
Storage and Warnings:

Store in a tight, light resistant, childproof container in a cool, dry place at room temperature away from heat and direct sunlight.

Refrigerate oral suspension.

Keep this and all medication out of reach of children and pets. Owners with low thyroid function should be cautious when handling and should avoid all skin contact.

Call your physician immediately if you accidentally take this product.

Potential side effects:
  • Any side effects usually occur within the first three months. Side effects may be reduced by beginning at a smaller dose and working up to a full therapeutic dose within a few months. If you notice any of the effects mentioned below, notify your veterinarian.
  • Decreased appetite, vomiting and lethargy. You may notice these within the first weeks of treatment.
  • Liver problems (including the above symptoms, plus dark urine, yellowing of the gums, skin or eyes), facial itching resulting in scratching or bleeding tendencies
  • Underlying kidney disease may become apparent
  • Rare cases of myasthenia gravis (severely weakened muscles, difficulty swallowing)
  • Unusually tired, fever (temperature over 103°F), bruising or bleeding
  • If you notice any of these symptoms or anything else unusual, contact your veterinarian
Can this drug be given with other drugs?
  • Yes, but possible interactions may occur with amitriptyline, beta-blockers, clomipramine, cyclophosphamide, digoxin, metoprolol, modified live vaccines, omeprazole, propanolol, theophylline and warfarin
  • If your pet experiences any unusual reactions when taking multiple medications, contact your veterinarian.

Contact your veterinarian immediately if pet receives more than the prescribed amount.

What else should I know?

Notify your veterinarian if your animal's condition does not improve or worsens despite this treatment.

As with all prescribed medicines, methimazole should only be given to the pet for which it was prescribed. It should be given only for the condition for which it was prescribed.

This is just a summary of information about methimazole. If you have any questions or concerns about methimazole or the condition it was prescribed for, contact your veterinarian.

(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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