To the Editor.— A study of the concentrations of Δ 9 -tetrahydrocannabinol (THC) and its metabolites found in plasma after the passive inhalation of marijuana smoke was done. This study was performed in conjunction with a study of the urinary excretion of cannabinoid metabolites after passive inhalation. In the latter study, the passive exposure to marijuana smoke was maximized. Four subjects simultaneously smoked four National Institute of Drug Abuse (NIDA) marijuana cigarettes, each containing 26.2 mg of THC, in a manner such that a maximum amount of smoke was released into a small, closed room. The experimental details and results of the urinary excretion experiment are presented in the preceding letter. On the second day of the three-day urinary study, a third passive smoker participated. The plasma samples obtained from this subject during the one-hour passive exposure were analyzed for THC by radioimmunoassay (RIA)1 and for THC and its major
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