Methimazole for Cats: Uses, Dosage, and Side Effects

PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.  

What Is Methimazole Used for in Cats?

Methimazole is a prescription anti-thyroid medication used for the management of hyperthyroidism in cats.  

Hyperthyroidism is a condition that occurs when there is excess thyroid hormone in a cat’s ​​system. The thyroid hormone helps regulate a cat’s metabolism. Symptoms of hyperthyroidism can include: 

• ​​Weight loss​​​ 

• ​​​Increased appetite​​​​ 

• Increased vocalization 

• Vomiting 

• Increased drinking 

• Increased urination 

Senior cats are more commonly diagnosed with the disease. 

Is Methimazole FDA-Approved for Cats? 

Yes, methimazole is FDA-approved for use in cats to treat hyperthyroidism. 

FDA-approved veterinary products include Felimazole tablets and Felanorm oral solution. The FDA has also approved ​the first generic methimazole coated tablets for cats, providing additional tablet options. 

Methimazole is FDA-approved for humans ​as ​​generic methimazole.​ Veterinarians may prescribe human-approved medications for cats when appropriate, which is known as extra-label or off-label use. 

Your veterinarian will determine whether methimazole is the right choice for your cat. 

Compounded Methimazole for Cats 

In certain circumstances, your vet may recommend a compounded formulation of methimazole. 

Compounded medications are prescribed if there’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if your pet has trouble taking pills in capsule form, the dosage strength is not commercially available, or the pet is allergic to an ingredient in the FDA-approved medication.  

Compounded medications are not FDA-approved. They are created by either a veterinarian or a licensed pharmacist on an individual basis to best suit a patient’s particular needs. 

Is Methimazole Safe for All Cats? 

​​No, methimazole isn’t safe for all cats. ​​It ​should​n’t ​be given to: 

• Cats with ​​​​​liver disease​, autoimmune diseases, kidney disease, or other blood abnormalities 

• ​​Pregnant​ or lactating cats 

• Cats allergic to methimazole, carbimazole​,​ or polyethylene glycol  

Giving methimazole with certain medications can result in health risks to your cat, so it’s important to discuss your cat’s medications, including vitamins and supplements, and medical conditions with your veterinarian. 

How Does Methimazole Work in Cats?

Methimazole helps control hyperthyroidism by reducing how much thyroid hormone the thyroid gland makes. Too much thyroid hormone speeds up a cat’s metabolism and causes ​hyperthyroid ​symptoms ​such as​ weight loss and increased appetite. 

Methimazole does not cure the condition, so it must be given consistently and long term to keep thyroid hormone levels in a healthy range and help manage symptoms. 

How To Give Methimazole to Cats

Follow the directions on the drug label or as provided by your veterinarian. 

​​Dosing is highly dependent on an individual cat’s needs. ​ 

What Happens If You Miss a Dose of Methimazole? 

Speak with your veterinarian about what to do if you forget to give a dose of methimazole. In general, your vet may recommend giving the dose when you remember​,​ unless it​’​s close to the time for the next scheduled dose. 

Do not give extra or double doses. 

Possible Side Effects of Methimazole in Cats

Side effects from methimazole typically occur within the first three months of use. 

The most common side effects are:  

• Not eating (anorexia)   

• Vomiting  

• Diarrhea 

• Depression 

• Mild to moderate changes in your pet’s white blood cell count, which typically self-resolve   

Rare side effects can include:  

• Severe facial itching   

• A decrease in ​whi​te ​​blood​ cells and/or​ platelet levels  

• Liver disease 

• Lowered levels of thyroid hormone in the body can also detect underlying kidney disease if present  

Call your veterinarian if you see any of these side effects in your cat. 

Human Side Effects 

​​​​Due to possible side effects, humans should never use medicine dispensed for their pets and pets should not be given any medicine dispensed for a human’s use. ​​​ 

​​​Methimazole frequently has dosages different from those prescribed for your pet by a veterinarian ​​and can affect the human thyroid if absorbed. 

To reduce exposure: 

• Do not break or crush tablets. 

• Wear gloves when handling methimazole or cleaning up spills, litter, urine, feces, or vomit from treated cats. 

• Wash your hands after handling the medication. 

• If you are pregnant or nursing, take extra precautions, and avoid direct contact when possible. 

If you accidentally ingest a pet medication, seek medical attention in person or call Poison Control at 800-222-1222. 

Monitoring Your Pet 

Before starting methimazole, cats should have blood and urine tests to check thyroid levels and overall health. 

After treatment begins, thyroid levels are typically rechecked at three and six weeks, then every few months once the dose is stable. Your veterinarian may recommend additional testing as needed. 

​​​When To Call Your Vet 

Call your veterinarian if you see any of the following, or if you have additional questions or concerns about the use of methimazole: 

• Severe side effects (above) 

• Your pet’s condition worsens or does not improve with treatment 

• You see or suspect an overdose 

What Happens If a Cat Overdoses on Methimazole?

One-time overdoses may cause the adverse reactions listed above. 

White blood cell changes, liver disease, low platelet levels, and other serious side effects can also be seen. 

If you suspect an overdose, immediately seek emergency veterinary care or contact an animal poison control center. Consultation fees often apply. 

• Pet Poison Helpline 855-764-7661  

• ASPCA Animal Poison Control 888-426-4435  

How To Store Methimazole for Cats

​​Keep the following storage tips in mind for this medication:​​​​​     ​​ 

• ​​Methimazole should be stored at controlled room temperatures of 68–77 F (20–25 C). Brief exposure to temperatures of 59–86 F (15–30 C) is acceptable.​​​ 

• ​​Compounded medications should be stored according to the compounding pharmacy’s label. ​ 

• Keep the container tightly closed to protect from moisture and light.  

• Keep out of reach of children and pets.