Cocaine levels in sweat patches decline over time

Sweat analysis may be useful for monitoring cocaine abuse. We measured sweat levels after 1 and 2.5 mg/kg of i.v. cocaine-d₅ and found that cocaine levels in the Pharmchek™ sweat patch declined over time. To assess the mechanism for the decline in cocaine levels, we spiked 48 sweat patches with cocaine-d₅ (100 ng as base/patch) and placed them on 8 cocaine-naive volunteers. Patches were removed at 1, 3, 8, 24, 48, and 72 hr after application. To investigate the possible loss of cocaine to the environment, 8 drug-free patches were placed over cocaine-containing patches. Cocaine and benzoylecognine (BE) levels were determined using GC/MS. Cocaine levels in the spiked patches declined over 72 hr (p=0.0002) with levels at 48-hr post application 30% less than those removed at 1 hr. Conversion to BE was detectable at low levels (<2%). Four (50%) of 8 cocaine-free patches placed on cocaine-containing patches became positive for cocaine during patch wear. These results suggest that cocaine is lost from patches by reabsorption across the skin and transfer to the environment. The low patch BE levels suggest that degradation by skin flora or spontaneous hydrolysis is a less likely explanation. We conclude that substantial losses of cocaine may occur from patches, decreasing their utility in monitoring illicit drug use.