Urinary Concentrations of Topically Administered Pain Medications

A common treatment for chronic pain is prescription of analgesics, but their long-term use entails risk of morbidity, addiction and misuse. One way to reduce the risk of abuse is prescribing of analgesics in a topical form. Physicians are urged to perform urine drug testing to ensure that patients are compliant with their medication regimens. However, there is little data on the efficiency of transdermal delivery for many analgesic drugs, and no data on expected urine drug levels. This study includes data from over 29,000 specimens tested for gabapentin, ketamine, cyclobenzaprine or amitriptyline used orally or topically. Gabapentin and amitriptyline concentrations were more likely to be below the limits of detection (25-40 ng/mL) in the urine of patients using them topically as compared with patients using them orally. Levels in gabapentin-positive topical specimens were much lower than in gabapentin-positive oral specimens (261 ng/mL vs >10,000 ng/mL). In contrast, ketamine and cyclobenzaprine were more readily detectable in the urine of topical users. Ketamine topical specimens were positive 12% more often than oral specimens, and mean topical specimen levels were 68-100% those of oral specimens. Cyclobenzaprine specimens were equally likely to be positive whether the dose was oral or topical, although mean levels after topical dosing were approximately 13-21% those after oral dosing. These findings are consistent with the reported percutaneous absorption efficiencies of gabapentin and ketamine, and are likely to be related to the absorption efficiencies of cyclobenzaprine and amitriptyline.