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Felimazole vs. Methimazole for Cats: Comparison Guide

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The most common endocrine disorder in cats is hyperthyroidism. Without medication, your cat will experience a wide range of symptoms and not feel very well. 

If you speak with your veterinarian, they will offer you a couple choices to treat your cat. Methimazole is a great treatment choice, and there are multiple options for giving your cat this medication. 

Read this article to decipher the differences between brand name tablets known as Felimazole vs. methimazole.

Felimazole Vs. Methimazole: A Quick Comparison Guide

Type of medicationAntithyroid thioamideAntithyroid thioamide
Active ingredient(s)MethimazoleMethimazole
Used to TreatHyperthyroidism (cats)Hyperthyroidism
Available FormsCoated tabletsTablets, liquid, transdermal gel
Prescription or OTCPrescriptionPrescription
Dosing Frequency1-2 times per day1-2 times per day
When Does It Start Working?Within 2 weeksWithin 2-4 weeks
Possible Side EffectsVomiting, decreased appetite, less activeVomiting, decreased appetite, less active
Safe for Long-Term Use?YesYes
FDA ApprovedYesYes

Although the chart highlights Felimazole and methimazole, there are two other common names for the medication. Tapazole is a brand name for methimazole in human medicine. Thiamazole is another name for the active ingredient methimazole.

Key Differences in Felimazole and Methimazole for Cats

Methimazole and Felimazole are similar in that the active ingredient is the same in both medications. However, the inactive ingredients are the main difference. Inactive ingredients are the components that make the medication effective but aren’t the therapeutic ingredient itself. 

Felimazole contains several ingredients including sugar, coloring, and beeswax that form the outer coating of the tablets.

Methimazole comes in a variety of forms including tablets, liquid, and a transdermal gel (for putting on the underside of the ear). The liquid and transdermal gel need to be compounded, or made by pharmacies who specialize in taking the original main ingredient and changing it into other forms. Felimazole is only available in tablet form.

Methimazole tablets only come in 5 mg, 10 mg, and 20 mg strengths (unless compounded). Felimazole comes in two sizes better suited for cats – 5 mg and 2.5 mg. Many cats start with a dose of 1.25 mg or 2.5 mg. In addition, the Felimazole tablets are smaller and have a sugar coating, making it easier to administer to some felines.

Felimazole and methimazole are FDA approved for treating hyperthyroidism in cats. However, Felimazole’s package has several warnings for people. Dechra, the company who makes Felimazole, felt that the warnings were necessary because if people get the medication on their hands and accidentally eat some of it, the medication will get into their systems. These warnings are not on generic methimazole because it is a medication formulated for people who need to take the medication for health reasons. 

However, whether you are giving your cat Felimazole or generic methimazole, you should take precautions when handling this medication. People should wash their hands with soap and water after giving their cat the medication, and wear gloves when cleaning urine, feces (poop), vomit, or litter from their cats. Gloves are also advised if a pill gets moist or broken. 

Despite these warnings, a recent study demonstrated that cat parents who gave their cats methimazole by mouth did not have traces of the medication in their urine samples following the study period [1].

Pros and Cons: Felimazole Vs. Methimazole

FelimazoleComes in smaller sizes that offer better dosing options for cats.

Contains a sugar coating that masks any unpleasant tastes, making administering tablets easier.
More expensive than generic methimazole.
MethimazoleMore cost effective than Felimazole.

Can be compounded into a liquid or transdermal gel. Transdermal application of methimazole has been proven to be effective when applied to the inner skin of the ear [2].
Regular tablets can be more difficult to administer to cats.

Methimazole pills start at 5 mg, which could make getting the right dose more challenging.

Effectiveness of Felimazole and Methimazole on Feline Hyperthyroidism

Overall, both Felimazole and methimazole are effective at treating hyperthyroidism in cats. Over 95 percent of cats will respond well to the medication [3], causing their thyroid levels to normalize and clinical signs of hyperthyroidism to go away. 

For oral methimazole, about 80 percent of cats will have normal thyroid levels in just two weeks [4]. For transdermal medication, it will likely take 4 weeks for your cat’s thyroid levels to normalize. 

Comparing Felimazole and Methimazole Side Effects

Both methimazole and Felimazole can cause very similar side effects in cats. Gastrointestinal issues, or side effects involving the stomach or intestines, can occur including vomiting, diarrhea, and decreased appetite. Lethargy, or decreased activity, may occur also. 

Older cats more frequently have underlying kidney disease, and the kidneys can worsen when taking methimazole or Felimazole. Less commonly, cats can develop liver disease and may even turn a yellow color on their skin and eyes. Severe itching of the face or issues with abnormal bleeding could occur as well.

If your cat takes this medication long-term, you may note side effects such as behaving withdrawn or depressed, weight loss, change in the fur coat, weakness, agitation, or diarrhea. Most of these side effects are transient, meaning they come and go.

The main difference between Felimazole and methimazole is if using transdermal methimazole, cat parents may notice redness or other changes in the skin of the ear where the medication is applied. Cats that receive transdermal methimazole are less likely to have gastrointestinal side effects than Felimazole or methimazole given by mouth.

Cost Comparison of Felimazole Vs. Methimazole

Felimazole is approximately twice the cost of methimazole tablets. Regardless, compared to the cost of other medications, Felimazole is relatively affordable at $15 to $20 per month. Methimazole transdermal or compounded liquid formulations can cost much more, approximately $50 per month.

Insurance may help with covering your cat’s medication, but most insurance companies do not cover pre-existing conditions. This means that if your cat was diagnosed with hyperthyroidism prior to purchasing insurance, the company may not cover the cost of the medication. The only way to know for sure is to call the insurance company you are interested in.

The Bottom Line

Overall, both Felimazole and generic methimazole are effective for the treatment of hyperthyroidism in cats. Both can also cause a myriad of side effects. 

The major difference is that methimazole can be made into liquid or transdermal formulations for pet parents who need an alternative to tablets. While Felimazole is a more expensive tablet than generic methimazole, it has a soft sugar coating that prevents cats from tasting the bitter medication and comes in a smaller size that keeps pet parents from having to cut it for dosing. 

As long as you’re treating hyperthyroidism, you can’t go wrong with either of these medications.


  1. https://pubmed.ncbi.nlm.nih.gov/35471087/ 
  2. https://pubmed.ncbi.nlm.nih.gov/24174499/ 
  3. https://journals.sagepub.com/doi/pdf/10.1177/1098612X16643252  
  4. https://pubmed.ncbi.nlm.nih.gov/26306818/