Psychoactive drugs are often widely used before tolerance and dependence is fully appreciated. Tolerance to cannabis-induced cardiovascular and autonomic changes, decreased intraocular pressure, sleep and sleep EEG, mood and behavioral changes is acquired and, to a great degree, lost rapidly with optimal conditions. Mechanisms appear more functional than metabolic. Acquisition rate depends on dose and dose schedule. Dependence, manifested by withdrawal symptoms after as little as 7 days of THC administration, is characterized by irritability, restlessness, insomnia, anorexia, nausea, sweating, salivation, increased body temperature, altered sleep and waking EEG, tremor, and weight loss. Mild and transient in the 120 subjects studied, the syndrome was similar to sedative drug withdrawal. Tolerance to drug side effects can be useful. Tolerance to therapeutic effects or target symptoms poses problems. Clinical significance of dependence is difficult to assess since drug-seeking behavior has many determinants. Cannabis-induced super sensitivity should be considered wherever chronic drug administration is anticipated in conditions like epilepsy, glaucoma or chronic pain. Cannabis pharmacology suggests ways of minimizing tolerance and dependence problems.