Gabapentin, a human therapeutic medication and an environmental substance transferring at trace levels to horses: a case report

Gabapentin is one of the most prescribed human medications and it is also available to be used recreationally in the United Sates. This is a case report about a Standardbred racehorse that raced in Ohio, won and tested positive for Gabapentin during screening. The laboratory of Analytical Toxiciology of the Ohio Department of Agriculture found suspected presence of Gabapentin and further confirmed the presence of the substance on post-race blood sample, the serum concentration reported was 273 pg/ml. This was a low concentration “positive”, similar situation have presented in Ohio and the suspicion was not intentional and random exposure of horses to environmental Gabapentin.

This positive result would implicate a payment of a fine and 15 days of disqualification. Because the substance was from unknown origin, horsemen were unable to take action to prevent this positive results. Similar to this, an amphetamine related substance was testing positive in the past and later discovered to be the result of the metabolic transformation of Levamisole, an equine anthelmintic treatment.

The main suspect was human excretion of Gabapentin to the environment, being the 11th most prescribed drug in the US and that it can be excreted by humans unchanged, this route for the chemically stable substance seemed likely. After further investigation, the cases of horses with low positives were linked to humans that had Gabapentin prescriptions. Alike incidents have occurred in California and there are reports of dogs that were prescribed this medication and had access to horse stables and have exposed horses.

To conclude it seems possible that Gabapentin is transferred to horses from the environment having humans as the source. The amounts transferring to horses are minimal, so these finds support the Ohio State Racing Commission on placing the screening limit of 8 ng/ml for Gabapentin in Equine Plasma. Allowing the horsemen and their animals to be under prescription or recreational medication without risking testing a low positive result during a race.

Brewer, K. et al. (2022) ‘Gabapentin, a human therapeutic medication and an environmental substance transferring at trace levels to horses: a case report’ Irish Veterinary Journal. 75(19)
DOI: https://doi.org/10.1186/s13620-022-00226-5