Establishment of residual cognitive and psychotic effects (effects present at the time that all active cannabinoids are eliminated from the body) putatively produced by prolonged heavy cannabis use is difficult, because of many confounding variables like slow elimination of active cannabinoids, lack of supervision during abstinence, poor use of well-matched control groups and the presence of withdrawal symptoms. Residual cognitive effects were observed in some but not in all tests after prolonged heavy cannabis use. The effects were mostly mild. The relationship of cannabis use, psychotic effects and schizophrenia was unclear; the cannabis conceivably gave relief, but it also appeared that cannabis caused schizophrenia in young people and (or) enhanced the symptoms, especially in young people poorly able to cope with stress or in whom the antipsychotic therapy was unsuccessful.