Category: Antithyroid agent
Other Names for this Medication: Tapazole®, Felimazole®
Common Dosage Forms: Veterinary: 2.5 mg & 5 mg sugar-coated tablets. Topical gels may be available from compounding pharmacies. Human: 5 mg & 10 mg tablets.

This information sheet does not contain all available information for this medication. It is to help answer commonly asked questions and help you give the medication safely and

effectively to your animal. If you have other questions or need more information about this medication, contact your veterinarian or pharmacist.

Key Information

X Used to lower the amount of thyroid hormones in animals.

X Available in different dosage forms, including a topical gel.X Special handling considerations for owners, especially

pregnant or nursing women.

X Most side effects (eg, vomiting, decreased appetite, or lethargy [tiredness; lack of energy]) occur in the first
3 months of therapy. Side effects may improve with a temporary, lower dose with gradual increase to desired dose.

How is this medication useful?

Methimazole is given to lower thyroid hormone levels. It is impor- tant to understand that methimazole does not cure an overactive thyroid but controls the disease if given to the animal for the rest of its life. Veterinarians may also prescribe methimazole for dogs to protect their kidneys while they are receiving cisplatin chemother- apy.

The FDA (U.S. Food & Drug Administration) has approved this drug for treating hyperthyroidism in cats. The FDA allows veterinarians to prescribe and use products containing this drug in other animal species in certain situations. You and your veterinarian can discuss why this drug is the most appropriate choice.

What should I tell my veterinarian to see if this medication can be safely given?

Many things might affect how well this drug will work in your animal. Be sure to discuss the following with your veterinarian so together you can make the best treatment decisions.
XXTell your veterinarian and pharmacist what medications

(including vitamins, supplements, or herbal therapies) you are giving your animal, as well as the amount and schedule of each.

XXTell your veterinarian about any conditions or diseases your pet may have now or may have had in the past.

When should this medication not be used or be used very carefully?

No drug is 100% safe in all patients, but your veterinarian will discuss with you any specific concerns about using this drug in your animal.

This drug SHOULD NOT be used in patients:
XXThat have had an allergic reaction to methimazole, carbimazole,

or polyethylene glycol.
This drug should be used WITH CAUTION in patients that:

XXHave anemia, liver disease, kidney failure, or immune system problems.

XXAre pregnant or nursing.
If your pet has any of these conditions or signs, talk to your veteri-

narian about the potential risks versus benefits.

What are the side effects of this medication?

Methimazole is usually well tolerated, but side effects can occur; most happen in the first 3 months of therapy.

Common but not serious side effects:


XXDecreased appetite or loss of appetite.

You do not have to be overly concerned if you see any of these unless they are severe, are persistent, or worsen. Contact your veterinarian if this happens.

Side effects that may be serious or indicate a serious problem:



XXDepression, lethargy (lack of energy).

XXSevere weakness (rare).

XXItching and scratching (particularly of the face) to the point that the animal causes bleeding.

XXSores (rare).
If you see any of these, contact your veterinarian immediately.

If my pet gets too much of this medication (an overdose), what should I do?

Overdoses of methimazole can cause vomiting, loss of appetite, sleepiness or sluggishness, weakness, fever, bruising, or sores. If you witness or suspect an overdose, contact your veterinarian or an animal poison control center for further advice. Animal poison control centers that are open 24 hours a day include: Pet Poison HELPLINE (855-764-7661) and ASPCA Animal Poison Control Cen- ter (888-426-4435); a consultation fee is charged for these services.

XXIf your animal has been treated for the same disease or condition in the past, tell your veterinarian about the treatment and how well it did or didn’t work.

XXIf your animal is pregnant or nursing, talk to your veterinarian about the risks of using this drug.

XXTell your veterinarian and pharmacist about any medication side effects (including allergic reactions, lack of appetite, diarrhea, itching, hair loss) your pet has developed in the past.

©2017 PharmaVet, Inc. ALL RIGHTS RESERVED. Plumb’s® Veterinary Medication Guides have not been reviewed by FDA Center for Veterinary Medicine.

How should this medication be given?

XXFor this medication to work, give it exactly as your veterinarian has prescribed. Do not cut or break tablets unless instructed
to do so by your veterinarian or pharmacist. It’s a good idea to always check the prescription label to be sure you are giving the drug correctly.

XXMethimazole may be given either with food or on an empty stomach. If your pet vomits or acts sick after receiving the drug on an empty stomach, try giving the next dose with food or a small treat. If vomiting continues, contact your veterinarian. Methimazole tablets taste bitter, so it may be necessary to hide them in food or treats.

XXCompounded liquid forms of this medication must be measured carefully. Your veterinarian or pharmacist can help by providing special measuring spoons or syringes.

XXMethimazole is often given as a topical gel (also called transdermal or PLO) that you rub into your pet’s ear flap twice daily. If you are using the gel form, wear disposable gloves and wash your hands after each use.

XXIf you have difficulty getting your animal to take the medicine, contact your veterinarian or pharmacist for tips to help dosing and to reduce the stress of medication time.

XXThis medication can be given for various lengths of time, often for the remainder of your pet’s life. Be sure you understand how long your veterinarian wants you to continue giving this medication. Prescription refills may be necessary before the therapy will

be complete. Before stopping this medication, talk to your veterinarian, as there may be important reasons to continue its use.

What should I do if I miss giving a dose of this medication?

If you miss a dose, wait and give the next dose when it is usually time to do so. Do not double up doses or give an extra dose.

How should I store this medication?

XXStore this medication in the original prescription bottle or an approved dosage reminder container (ie, pill minder) at room temperature, out of direct sunlight. If your veterinarian or pharmacist has made (compounded) a special formulation for your animal, follow the storage recommendations and expiration date for the product.

XXKeep away from children and other animals.

Can handling this medication be hazardous to me, my family, or other pets?

Pregnant women or women who may become pregnant, nursing mothers, and people with low thyroid hormones should avoid han- dling methimazole. If this is not possible, disposable gloves must be worn while handling this medication or the litter or bodily fluids of treated animals. Do not reuse gloves. Once used, they should be thrown away.

How should I dispose of this medication if I don’t use it all?

XXDo not flush this medication down the toilet or wash it down the sink. If a community drug “take-back” program is available, use this option. If there is no take-back program, mix the drug with coffee grounds or cat litter (to make it undesirable to children and animals and unrecognizable to people who might go through your trash), place the mixture in a sealable plastic bag to keep it from leaking out, and throw the bag out with the regular trash.

XXDo not save leftover medication for future use or give it to others to use.

What other information is important for this medication?

XXAnimals receiving methimazole require close monitoring with blood work to make sure the dosing is correct and that the pet’s body is tolerating the medication well. The frequency of blood work will decrease after the first year unless there are new signs or side effects.

XXKittens should be placed on a milk replacer after receiving colostrum from mothers on methimazole.

XXUse of this drug may not be allowed in certain animal competitions. Check rules and regulations before entering your animal in a competition while this medication is being administered.

If you have any other questions or concerns about this medica- tion, contact your veterinarian or pharmacist.

©2017 PharmaVet, Inc. ALL RIGHTS RESERVED. Plumb’s® Veterinary Medication Guides have not been reviewed by FDA Center for Veterinary Medicine.