Methimazole-Handling Precautions for Cat Owners

Methimazole is considered by most veterinarians to be the antithyroid agent of choice when using drugs to control feline hyperthyroidism (1,2).

For many years, veterinarians prescribed methimazole to cats "off-label," using either a generic or brand-name product (Tapazole and Northyx) designed for human use (3-5).  A methimazole product specifically licensed for treatment of cats with hyperthyroidism (Felimazole, Dechra Veterinary Products) was only first released in 2009 (6).

In addition to methimazole tablets, transdermal methimazole gels are also commonly used in hyperthyroid cats, particularly in fractious cats or in those that develop GI side effects from the oral methimazole drug (7,8). Such transdermal preparations are not FDA-approved but are widely available through compounding pharmacies.

Is Methimazole a Safe Drug to Handle?
Label warnings on Felimazole (9), the form of the drug actually made and licensed for cats, points out the potential risks of methimazole in humans handling the drug. Many owners and veterinarians are not aware of the potential dangers associated with the "casual handling" of this drug.

The Felimazole label has a section on “Human Warnings” that reads as follows (9):

Human warnings

Not for use in humans. Keep out of reach of children. For use in cats only. 

Wash hands with soap and water after administration to avoid exposure to drug. 

Do not break or crush 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. Methimazole is a human teratogen and crosses the placenta concentrating in the fetal thyroid gland. There is also a high rate of transfer into breast milk. Pregnant women or women who may become pregnant, and nursing mothers should wear gloves when handling tablets, litter or bodily fluids of treated cats. Methimazole may cause vomiting, gastric distress, headache, fever, arthralgia, pruritus, and pancytopenia. In the event of accidental ingestion/overdose, seek medical advice immediately and show the product label to the physician. 

What's the Issue Here?
The major issue is oral absorption of residual drug (powder, dust) on a person's hands rather than direct absorption through the human skin. However, methimazole potentially can be absorbed through human skin as well, since the transdermal product in well-absorbed in most cats.

Methimazole is a human teratogen (i.e., the drug may cause birth defects) and crosses the placenta concentrating in the fetal thyroid gland. Besides being a safety issue in pregnant women or women who may become pregnant, the drug should not be handled by lactating women because methimazole is transferred in breast milk at a high rate.

The same precautions pertain regardless of what methimazole product is used, i.e., the veterinary brand-name Felimazole preparation or a human brand-name or generic product.  The methimazole tablets should not be cut with our bare hands. We want to wash our hands after administering the medication to minimize skin and oral contamination; repeated and chronic exposure could pose a risk. Special attention should be given to the potential for unwitting human exposure through cat’s bodily fluids and excretions, including the feces, urine, and vomitus.

Bottom Line: 

• Methimazole, in any form, should be handled with some caution. 
• Wash hands with soap and water after handling drug. 
• Do not break, cut, or crush tablets. 
• Wear protective gloves when handling litter, feces, urine, or vomit of treated cats.
• Wear gloves when handling broken or moistened tablets. 
• Wash hands after contact with the litter of treated cats.
• Wear gloves or finger cots when applying transdermal methimazole.
• Keep all forms of methimazole out of reach of children.
• Pregnant women and nursing mothers should not handle methimazole, if possible 

These warning may be overkill, but it's always best to be careful, especially if one is pregnant. Absorption through intact skin is probably minimal, but transfer of the "powder" from the methimazole tablet or transdermal gel into the mouth, nose, or eyes might be significant over time. So never forget to washes you hands after touching the methimazole tablets or gel.

References:

1. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110. 
2. Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012;571-592. 
3. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150-157. 
4. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22–28.
5. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North American Small Animal Practice 2007;37:775-788.
6. VIN News Service. FDA green lights Felimazole to treat feline hyperthyroidism. June 11, 2009.
7. Hoffmann G, Marks SL, Taboada J, et al. Transdermal methimazole treatment in cats with hyperthyroidism. Journal of Feline Medicine and Surgery 2003;5:77-82.
8. Sartor LL, Trepanier LA, Kroll MM, et al. Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism. Journal of Veterinary Internal Medicine  2004;18:651-655.
9. Felimazole-Coated Tablets. Product insert. Available at: www.dechra-us.com/files/dechraUSA/downloads/Product%20inserts/Felimazole.pdf