A new study finds that one-third of all drivers pulled over for suspicion of impaired driving tested positive for illicit drugs even though they were under the legal limit for driving with alcohol in their systems, Medical News Today reported Dec. 26.

Irish researchers examined 2,000 blood and urine samples collected from drivers who had been pulled over by police for suspected drunk driving. They found that one in three samples taken from drivers found to be under the legal alcohol limit had one or more illicit drugs in their system -- a far higher rate than among drivers who were found to be legally intoxicated from alcohol consumption.

Marijuana was the most common drug detected other than alcohol.

The authors recommended that all drivers suspected of impairment be tested for illicit drugs as well as alcohol. "Too little attention has been paid to the adverse effects of drugs on driving, but drugged driving can be as dangerous as drunken driving," they wrote.

The study appears in the Dec. 1 issue of the journal Injury Prevention.