This chapter relates to illicit drugs such as cannabis, LSD, heroin and cocaine as well as substances such as glues and solvents. As noted in Chapter 4, the latter are not ‘illegal’ in the same sense as substances covered by the Misuse of Drugs Act (1971). It is difficult, as outlined in the two preceding chapters, to measure the extent of the adverse consequences of legal drug use. It is even harder to measure the adverse consequences of using illicit drugs, the very use of which is only partly evident. Accordingly, only a partial picture is available. As already noted, though many young people do at sometime use illicit drugs, most do so only on a strictly temporary and limited basis. Regular or ‘heavy’ use is restricted to a small minority. It is therefore not surprising that most of those who use illicit drugs do not come to the attentions of ‘official agencies’ because of their use of these substances. Those who do come to such attention are particularly likely to be those who use drugs most often.