Carisoprodol, meprobamate, and driving impairment

This paper considers the pharmacology of the centrally acting muscle relaxant carisoprodol, and its metabolite meprobamate, which is also administered as an anxiolytic in its own right. Literature implicating these drugs in impaired driving is also reviewed. A series of 104 incidents in which these drugs were detected in the blood of drivers involved in accidents or arrested for impaired driving was considered, with respect to the analytical toxicology results, patterns of drug use in these subjects, the driving behaviors exhibited, and the symptoms observed in the drivers. Symptomatology and driving impairment were consistent with other CNS depressants, most notably alcohol. Reported driving behaviors included erratic lane travel, weaving, driving slowly, swerving, stopping in traffic, and hitting parked cars and other stationary objects. Drivers on contact by the police displayed poor balance and coordination, horizontal gaze nystagmus, bloodshot eyes, unsteadiness, slurred speech, slow responses, tendency to doze off or fall asleep, difficulty standing, walking or exiting their vehicles, and disorientation. Many of these cases had alcohol or other centrally acting drugs present also, making difficult the attribution of the documented impairment specifically to carisoprodol and meprobamate. In 21 cases, however, no other drugs were detected, and similar symptoms were present. Impairment appeared to be possible at any concentration of these two drugs; however, the most severe driving impairment and most overt symptoms of intoxication were noted when the combined concentration exceeded 10 mg/L, a level still within the normal therapeutic range.