Altogether, 29 percent of the drivers had evidence of alcohol, marijuana, cocaine, prescription or nonprescription stimulants, or a combination of these, in either blood or urine. Cannabinoids were found in 15 percent of the drivers' blood or urine, nonprescription stimulants such as phenylpropanolamine were found in 12 percent, prescription stimulants such as amphetamine were found in 5 percent, cocaine metabolites were found in 2 percent, and alcohol was found in less than 1 percent. These results provide the first objective information about the use of potentially abusive drugs by tractor-trailer drivers. The extent of driver impairment attributable to the observed drugs is uncertain because of the complex relationship between performance and drug concentration. (Author abstract)