As a result of advances in fields such as medical scanning and neurobiology, it is now possible to get a better idea of how drugs affect the brain. This report, submitted by Christian Cabal, summarises our knowledge about the effects of drugs on the brain. The rapporteur stresses the difficulty of drawing too general conclusions from this work. The effects of any particular drug vary widely between individuals and according to the way in which they are administered, and although there are hard drugs - such as heroin - which are always harmful, there are now "hard" ways of taking soft drugs, showing that the latter term has come to have little meaning. In this paper, readers will not find answers to questions about the criminalisation of drugs, for that is a separate issue, but they will at least see convincing evidence about the hazards of these substances.