Some of the factors considered in this thesis are discussed: prevalence of self-reported consumption by security level, types of substances consumed and frequency of use, motivations and perceived psychological state, reported perception of guards' tolerance, and subject-reported drug availability. The study was undertaken to attempt to explain the presence of psychoactive drugs in prison based on past history of use prior to incarceration and use of drugs to adjust to prison life. Drug use in prison was found to be much higher for drug users than for alcohol users. Guards' tolerance was found to be lower for hard drugs and alcohol than for cannabis. Drugs were easy to obtain in both minimum and maximum security prisons, though there was found to be less consumption at minimum security prisons. Results of the study indicate that strategies used for interrupting the supply of drugs or punishing users should be reconsidered, as the measurement of abandoning drug use in prison does not represent a cure, and incarceration can also be considered to influence inmates to behave in less harmful ways to the individual and thus to society. 1 Table, 12 notes