Will Felimazole® Replace Methimazole?

Posted on August 1, 2009

You’ve heard about methimazole for the treatment of feline hyperthyroidism.  Well now its time you hear about Felimazole®.  Will Felimazole® cure feline hyperthyroidism and replace the use of methimazole?

Our guess is you probably won’t see too many well informed veterinarians lining up to prescribe this recently approved (May 2009) drug for use in cats, by the U.S. Food and Drug Administration.  That is, except for veterinarians in the United Kingdom who will need to use Felimazole® exclusively since it was approved there.

The active ingredient in Felimazole® (Dechra Pharmaceuticals) is methimazole.  No typo here, that’s right.  Felimazole® is the same medication that has been prescribed by veterinarians for decades for the treatment of hyperthyroidism in cats, except we’ve been using it off-label and calling it methimazole.  It works by reducing production of T3 and T4.  In safety and efficacy studies conducted by the makers of Felimazole® they found the most frequent side effects to be vomiting, anorexia, lethargy, skin lesions, and diarrhea.  Acute death was reported in 4 cats and unmasking of renal disease occurred in a few cats.

So why did Dechra invest so much money into getting a drug approved for use in animals that is already used by most veterinarians?  Well it turns out any drug that is licensed for use in an animal for a specific disease must preferentially be used in the UK over off-label options.  So UK veterinarians, by law, are not allowed to prescribe methimazole since Felimazole® is available.  In the United States veterinarians can continue to prescribe whatever drug they feel is appropriate.  Presumably, Dechra will look to use the approval of their product to a marketing advantage over other options.  Our best guess is that most clients and veterinarians will choose the more competitively priced methimazole until or unless any difference between the two is shown (which seems unlikely). Only time (and marketing) will tell.

For more information about hyperthyroidism in cats, be sure to check out the Hyperthyroidism PowerPage in this month’s Veterinary Examiner.