A procedure is outlined for comparing dependence potential and acute toxicity across a broad range of abused psychoactive substances. Tentative results, based on an extensive literature review of 20 substances, suggested that the margin of safety ("therapeutic index") varied dramatically between substances. Intravenous heroin appeared to have the greatest risk of dependence and acute lethality; oral psilocybin appeared to have the least. Hazards due to behavioral deficits, perceptual distortion, or chronic illness were not factored into the assessments.