Depressants are a class of drugs which have different effects than stimulants, but are just as, or more, dangerous to the driver using them. Depressants slow down the activity of the nervous system. Besides alcohol, these drugs include inhalants, tranquilizers such as Valium, and sedatives such as barbituates. Most depressants may be obtained legally. Again, the effect on driving is heavily dependent on the dosage of the drug. The symptoms include sluggishness, lack of motor coordination, and relaxation. These symptoms are obviously detrimental to driving skills, and can easily make driving deadly. The effects of alcohol, a depressant, will be discussed in great detail in the following chapter.

Hallucinogens are related to depressants, but the effects are different. Of all the kinds of drugs, these are probably the most dangerous when taken before driving. Hallucinogens, as the name indicates, create altered perceptions. Hallucinogens include such drugs as LSD, and mescaline. These drugs are overwhelmingly illegal. Since these drugs directly alter the senses, the driving process becomes mostly guesswork, and extremely perilous. Marijuana is usually classified as a hallucinogen, though its effects vary. Though not typically as dangerous as other hallucinogens, it nonetheless makes driving a dangerous task. PCP is another drug that is hard to classify. It produces hallucinations, making it a hallucinogen, but it also detaches the mind from the body and sensory inputs, making it a dissociative. Other effects include provocation of violence, and paranoia.