The hangover effect of a single marijuana cigarette can interfere with a pilot's ability to land an airplane -- even 24 hours after the joint was smoked.

Researchers at Stanford University and the Veterans Administration used computers to measure the ability of 10 airplane pilots to perform landing maneuvers after smoking a cigarette containing 19 milligrams of tetrahydrocannabinol (THC), the active ingredient in marijuana. That's the equivalent of a "strong social dose" of pot, the researchers say.

The 10 test subjects -- all experienced private pilots and pot smokers -- were tested in a computerized flight simulation laboratory at the Veterans Administration Medical Center in Palo Alto, Calif. They were tested about one hour, four hours and 24 hours after smoking the marijuana.

At each interval, the study found, the pilots showed "significant impairments" in a variety of tasks involved in landing an airplane, compared with pre-pot tests. For example, they had more trouble manipulating the ailerons -- the movable wing parts that provide side-to-side control of the plane -- and the elevators, which provide vertical control.

They also were more likely to deviate from their proper landing approach and to land off-center, the study found.

"In actual flight, where there is wind and turbulence, such errors can easily lead to crashes," say the researchers in the American Journal of Psychiatry.

The impairment after 24 hours occurred even though the pilots themselves felt no drug hangover and reported normal alertness and mood -- suggesting that a marijuana smoker's own evaluation of his or her ability to handle a job safely may not be reliable.

The study's findings have implications far beyond the airplane industry and apply to any workers operating complex heavy machinery, says Stanford psychiatrist Dr. Jerome Yesavage, who directed the study.