Discriminative stimulus effects of diazepam in humans.

Humans were trained to discriminate between 10 mg of diazepam (DZ) and placebo. Subjects reported to the laboratory in the morning several times a week for a total of 23 sessions. They filled out subjective effects questionnaires, ingested a capsule and then were free to leave. They also filled out questionnaires 1, 3 and 6 hr after leaving. During the first four sessions the drugs were identified to the subject before ingestion by letter code. During the next seven sessions the procedure was the same except the capsules were not identified to the subject. Six hours after receiving the capsule, subjects telephoned the experimenter to report their identification. When correct, they received a monetary bonus. If the identification was correct on five of the seven sessions, subjects entered the third phase. This phase had six additional training sessions as described previously. During the other six sessions that were intermixed, subjects received capsules that contained test drugs. Sixteen of 18 subjects learned the discrimination and 14 entered the third phase. They identified 2 mg of DZ, 1 mg of lorazepam and 10 mg of d-amphetamine as placebo and 2 mg of lorazepam as 10 mg of DZ. Over half of the subjects identified 5 mg of DZ and 50 mg of pentobarbital as 10 mg of DZ. DZ (10 mg) produced significant time-related changes on several subjective effect measures and these effects were typical of those produced by sedative-like drugs. The subjective effects of the test compounds were largely correlated with the drug identification. These results indicate that 10 mg of DZ can function as a discriminative stimulus in humans and this discrimination is dose-related and pharmacologically specific.