Transitions in drug abuse (going from one substance to another and from one form of administration to another) has been scarcely studied in Latin America. In this project 96 people in treatment, aged 14 to 50, were interviewed in seven Colombian cities. Five kinds of transition were identified: general, cumulative, substitutive, reverse and negative, occurring at nine different points; 113 transitional patterns (combinations of two substances) were found, 23 common to men and women, 10 exclusive to women and 80 exclusive to men. Contrary to all expectations, 21 cases of heroin users appeared in this sample, mainly among upper- and middle-class users, more in men than in women, and in very young people: 11 out of 21 were under 20 years of age. The 10 main transitions only involve four substances: marihuana, cocaine, inhalants and tranquillizers, and are most common amongst those under age 23. The youngest use marihuana, cocaine, tranquillizers, glues, ecstasy, poppers and heroin; the eldest prefer cocaine, basuco and marihuana. Mixtures, changes of substance and changes of from of administration are always associated with a search for more powerful effects. Many youngsters believe that marihuana is either harmless or beneficial, because it can cure cancer caused by smoking tobacco. Heroin is considered the most dangerous drug because of its bad effects on all levels. The main limitations of this study were: the small number of participants, the even smaller number of women, and the fact that only people in treatment were interviewed.