Positive Warning: Detectable Levels of Levamisole Last for Days to Weeks 

A horse head looking out of a stall. Horses receive levamisole, a controlled substance.
Levamisole is commonly used to treat pharyngeal lymphoid hyperplasia in racehorses, but it is possible for horses to test positive for the substance weeks after a multiple-day dosing regimen. Getty Images

Levamisole is a medication occasionally used off-label to treat equine protozoal myeloencephalitis (EPM) and pharyngeal lymphoid hyperplasia, the latter of which is relatively common in young racehorses. It is a Class II Foreign Substance under the ARCI (Association of Racing Commissioners International) and a Controlled Medication Substance under HISA (Horseracing Integrity and Safety Authority). Further, two metabolites of levamisole—aminorex and pemoline—are stimulants and therefore categorized as banned substances. Yet, racehorses have tested positive for these metabolites, prompting further investigation into the detection of levamisole in blood and urine samples following a multiple-day dosing regimen. 

Study in Racehorses

“The study is important because we are all taking great pains to have the highest possible standards in equine sports,” relayed Emma Adam, DVM, PhD, Dipl. ACVIM, Dipl. ACVS, from the University of Kentucky’s Gluck Equine Research Center, during her presentation at the 2022 AAEP Convention. 

In the study, six Thoroughbred geldings recently retired from racing were administered 500 mg levamisole HCl in an oral compounded paste. Horses were dosed twice daily for a total of 13 doses.

Blood and urine samples were collected over a 28-day period starting with a pre-dosing sample. Intensive testing was performed immediately after the first and following the last dose of levamisole. In addition to levamisole, and its two metabolites aminorex and pemoline, compound II was measured. The detection of Compound II appears to be indicative of levamisole administration.  

Levamisole Half-Life

Adam reported that the half-life of levamisole was highly variable between study horses. 

“Overall, serum levamisole was still above the LOD in five of the six horses between days 3 and 14 of the study,” relayed Adam. 

“One horse remained above the LOD for levamisole on the final blood sample drawn on day 28, which was 21 days after the last dose was administered,” he said.

Compound II was below LOD 14 days after the last dose of levamisole was administered in all six horses. Neither aminorex nor pemoline were detected in blood samples. 

In urine, levamisole levels were below the LOD in all six horses six days after the last dose. Aminorex was below the LOD three days after the final dose. Compound II was above the LOD on the final day of sampling, 21 days after the final oral levamisole dose in four of the six horses where Day 28 samples were obtained. In the two other study horses where their final samples were 14 days after the last dose, one had urine Compound II above the LOD and one was below LOD. Pemoline was not detected in any urine samples. 

“This study shows that levamisole has a highly variable clearance time and may be detected in both serum and urine up to 21 days following 7 days of administration” concluded Adam.

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