It is not unusual for treatment of eye problems to require topical medication for extended periods. A Swiss study looked at the possibility that topical ophthalmic treatment with corticosteroids could end up in the systemic circulation and thereby lead to positive drug testing results [Stucki, M.; Voegel, C.D.; Binz, T.M.; et al. Systemic detectability of dexamethasone and prednisolone after eye drop application in horses. Equine Vet Journal Jan 2021; doi: 10.1111/evj.13418]. This has particular relevance for horses competing in sporting venues that ban any drug use, the Federation Equestrian International (FEI) as one example.
Eye inflammation responds well in many cases when treated with topical corticosteroids—the most commonly used drugs are dexamethasone and prednisolone. These medications are often tapered down over time to mitigate flares until ocular inflammation resolves.
In this study, five horses received dexamethasone eye drops and six horses received prednisolone eye drops—they were all medicated in one eye three times daily for two weeks. Systemic absorption occurs via the cornea, conjunctiva and nasal mucosa along the nasolacrimal duct. To measure for traces of corticosteroids, blood draws for serum occurred periodically over 15-120 minutes following topical eye administration, and urine was collected when the horse urinated.
- One day after cessation of dexamethasone eye drops, levels returned to baseline, i.e., below the limit of detection (LOD) in both serum and urine.
- Prednisolone returned to baseline in urine on the day following cessation of treatment.
- Prednisolone remained identifiable in serum until one week after cessation of treatment.
The authors summed up the relevant results to serve as guidelines for competition horses treated with corticosteroid eye drops: “All samples tested negative (below LOD) for dexamethasone one day and for prednisolone one week after treatment cessation.”