Manufacturers of moxidectin gel have changed the drug's label to indicate that it is for oral use in horses and ponies 6 months of age and older. The drug is used to control various species of internal parasites.

Since 1997, the label has specified that the drug is approved for use in horses and ponies 4 months of age and older, as long as the animals are not intended for food. A routine analysis by the Food and Drug Administration of postmarketing adverse drug event data for the product, however, revealed that horses less than 6 months of age have a higher incidence of adverse reactions, including ataxia, apparent depression/lethargy, and recumbency. For example, between 1997 and 2002, there were 111 reports of ataxia for horses less than 6 months of age compared with 196 reports for horses of all ages. The reactions are thought to be correlated with overdosing.

These same adverse reactions were also more common in horses weighing less than 250 pounds and older horses that were thin and debilitated. As indicated on the label, the body weight of all treated horses should be carefully estimated to preclude overdosing, and use of the product in thin and debilitated horses should be avoided. Fort Dodge Animal Health manufactures Quest (moxidectin) 2.0 percent Equine Oral Gel.

The FDA collects adverse event information on all drugs and routinely evaluates these data. Companies, veterinarians, or the general public can report adverse events to the FDA Center for Veterinary Medicine's Division of Surveillance. For more information, visit www.fda.gov/cvm/index/ade/adereporting.htm or call (888) FDA-VETS.