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Ten pilots smoked, in counterbalanced order on a double blind basis, a social dose of marihuana (.09 mg/kg delta-9-tetrahydrocannabinol) and a matched placebo after being trained to fly a specific flight sequence on an ATC-510 flight simulator. In contrast to placebo, marihuana caused a gross decrement in flying performance, with increased prevalence of major errors, minor errors, altitude deviations, heading deviations, and radio navigation errors. These effects of active marihuana persisted for at least 2 hr and generally had disappeared by 4 to 6 hr after marihuana administration.

This study finds evidence for 24-h carry-over effects of a moderate social dose of marijuana on a piloting task. In separate sessions, nine currently active pilots smoked one cigarette containing 20 mg of delta 9 THC and one Placebo cigarette. Using an aircraft simulator, pilots flew just before smoking, and 0.25, 4, 8, 24, and 48 h after smoking. Marijuana impaired performance at 0.25, 4, 8, and 24 h after smoking. While seven of the nine pilots showed some degree of impairment at 24 h after smoking, only one reported any awareness of the drug's effects. The results support our preliminary study and suggest that very complex human/machine performance can be impaired as long as 24 h after smoking a moderate social dose of marijuana, and that the user may be unaware of the drug's influence.