Dogs on drugs: FDA warns of dangerous drug interaction

By Christie Keith

June 26, 2008

Dogs who are taking the flea preventive drug Comfortis (spinosad) at the same time they are being given high dosages of the drug ivermectin, such as those used in the treatment of demodetic mange, are at risk of ivermectin toxicity.

The maker of the drug, Eli Lilly's companion animal health division, does not believe that there is any risk to using the regular heartworm-prevention dose of ivermectin with Comfortis; the daily dosages used to treat some stubborn cases of demodetic mange are as much as 100 times the monthly dosage used to prevent heartworm infection. In an informational release, the company cited a supporting field study that involved hundreds of dogs (PDF file):

The administration of Comfortis and approved canine formulations of ivermectin at doses labeled for heartworm prevention has been tested and shown to be safe, including in a North American field trial involving over 450 dogs that were required to be on monthly heartworm prevention throughout the three-month study. Laboratory work has found that, even at doses of 5 times the monthly dose of spinosad combined with 10 times the monthly dose of milbemycin oxime in ivermectin-sensitive collies, there were no signs of neurotoxicity (Sherman et al., publication pending).

The FDA Center for Veterinary Medicine is advising that the two drugs not be prescribed at the same time.


Ivermectin toxicity can be fatal and requires immediate veterinary care. Early signs include vomiting, weakness, drooling, tremors and coma. Dogs also often become blind, although this is usually reversible.

With aggressive veterinary care including hospitalization with round-the-clock nursing, most dogs will recover.

High-dose use of ivermectin is considered “extra-label,” which means a use of an approved drug to treat an illness for which it has not been approved. Such use is legal under the Animal Medicinal Drug Use Clarification Act of 1996; without extra-label prescribing, half or more of the drugs used in veterinary medicine every day would be unavailable, including most antibiotics.

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