Horsemen Reminded To Avoid Using Products Containing Levamisole and Tetramisole

Levamisole and tetramisole are sold as wormers for farm livestock and are also sometimes marketed as “immune stimulants” or “immune modulating” substances over the internet and in tack shops. Levamisole is classified by the Association of Racing Commissioners International’s Uniform Classification Guidelines as a Class 2, Penalty Class B substance while tetramisole is not currently classified.

There is very strong evidence in the scientific literature, however, that the administration of levamisole, and potentially tetramisole, to horses results in production of aminorex and pemoline as metabolites. Aminorex and pemoline are both Penalty Class A substances, which carry a minimum one-year suspension if detected in a post-race sample. Levamisole was linked to a significant number of aminorex positive tests in 2007 and advisories were issued to horsemen to avoid levamisole administration at the time.

Despite the advisories, there have been several positive tests for levamisole in the United States over the last 18 months. Because of the potential for severe penalties resulting from the detection of the metabolites associated with levamisole and tetramisole administration, MTHA would like to remind horsemen to avoid administering any product containing these drugs as worming agents and to pay particular attention to the ingredient list for any product being marketed as an immune system “boost.”

Additionally, there is no known “safe” withdrawal period for levamisole or tetramisole, so horsemen are advised to use alternative deworming drugs to avoid inadvertent positive tests.