There is general agreement that at the usual levels of social usage of marijuana, the subjective effects are alteration of time and space perception, sense of euphoria, relaxation, well being and disinhibition, dulling of attention, fragmentation of thought, impaired immediate memory, an altered sense of identity, exaggerated laughter, and increased suggestibility. Higher dosages produce more pronounced thought distortions. The physiological changes accompanying marijuana use at typical levels of social usage are relatively few. They include an increased pulse rate and reddening of the eyes at the time of use. Regarding intellectual and motor performance, the more complex and demanding the task to be performed, the greater is evidence of degree of impairment. Immediate and short-term memory is hampered. The diminishment of driving skills is similar to that for the influence of moderate amounts of alcohol. Currently, there is no evidence that marijuana use affects fetal development. There is some concern that marijuana smoked in large quantities may have carcinogenic effects similar to those associated with tobacco smoking. More must be learned about the interactions between marijuana and a wide range of other drugs.