For marijuana, passive inhalation, unknowing oral ingestion, and the use of Marinol are discussed. Data are presented on the concentration of THC and its precursors, acid-A and acid-B, in illicit marijuana and the extent of extraction of THC in boiled tea or cooked products. For cocaine, passive inhalation and passive exposure issues are reviewed. For opiates, poppy seed ingestion and guidelines for exclusion of poppy seeds as a cause for a positive test are discussed. For amphetamines, issues such as the presence of other phenethylamines, l-methamphetamine (Vicks' inhalers), and other prescription drugs are discussed. Although passive inhalation of methamphetamine and phencyclidine is theoretically possible, no data were available. Figures, tables, references