Three horses died after receiving compounded drugs that appear to have had about 20 times the pyrimethamine indicated on the label.

High doses of pyrimethamine also can cause seizure, fever, and collapse.

Food and Drug Administration officials warned horse owners and veterinarians that the deaths were connected with 60-mL syringes of oral paste sold by Rapid Equine Solutions to treat equine protozoal myeloencephalitis. The product label indicated the product contained toltrazuril at a concentration of 416 mg/mL and pyrimethamine at a concentration of 17 mg/mL.

The deaths involved two horses in Maine and one in Ohio. Rapid Equine Solutions, which is based in Aston, Pennsylvania, learned of the adverse events and started a recall May 24, according to the FDA warning.

About five years earlier, other drugs compounded with pyrimethamine and toltrazuril were connected with four deaths and six illnesses among horses in Kentucky and Florida, according to an FDA warning published in May 2014. A paste and an oral suspension, both from Wickliffe Pharmacy of Lexington, Kentucky, were connected with the adverse events, and at least one of the two lots contained higher concentrations of pyrimethamine than indicated on the labels.

In the 2014 and 2019 warnings, FDA officials noted that the compounded products were not FDA-approved animal drugs. Both warnings also state that toltrazuril is not FDA approved for use in horses.

FDA investigation focuses on compounded drugs (July 1, 2014)