This study investigates the effects of alcohol, drug intoxication and withdrawal symptoms on the mental state of criminal suspects and the nature of their confession. A total of 359 sentenced prisoners were approached on admission and 96% agreed to participate in the study. They completed a specially designed Confession Questionnaire, which asked them questions about their reasons for confessing to the police, their attitude towards their confession, their mental state at the time of the confession and the extent to which they had been under the influence of alcohol and drug intoxication at the time of crime and police interview. The confession rate for the sample was very high (92%), with serious traffic violators having the highest confession rate (95%) and sex offenders the lowest (83%). The findings illustrate that the “perception of proof”, “internal need to confess” and “external pressure” are the main reasons why suspects confess. However, subjects were identified who had experienced a typical “prisoner's dilemma” phenomenon during the police interview because of a co-defendant. Alcohol and drug intoxication was very commonly reported both at the time of the offence and the police interview, but these were related to the nature of their offence. Although intoxication and withdrawal symptoms did not appear to seriously impair their coping during the police interview it was consistently reported as having made them confused.