Marijuana
Research has shown that even typical social doses of marijuana can affect concentration, judgment and the sensory and perceptual skills needed for careful driving. People who are under marijuana's influence have impaired sensory and perceptual abilities.
Stimulants
While heavy amphetamine use will keep drivers awake and active for long stretches of time, it will also make them less coordinated, edgy, and as one accident study found, four times more likely to be involved in a car crash. Research shows that typical social amounts of cocaine can produce lapses in attention and concentration.
Although caffeine can help the drowsy driver stay alert, it can't make the drunk driver sober. Studies show that ordinary amounts of caffeine do not improve an inebriated subject's driving skills.
Tranquilizers and Other Sedative-Hypnotics
The sedative-hypnotic drugs, including barbiturates, are powerful depressants that calm people down or help them sleep. Sleepy or over sedated drivers, however, are not good drivers.