Polydrug use and implications for longitudinal research: ten-year trajectories for heroin, cocaine, and methamphetamine users.

A typical approach to categorizing substance users for epidemiologic purposes or to identify substance use problems at treatment admission is by indicating the primary substance used and/or for which treatment is sought. But does such singular focus on the primary drug limit the validity of conclusions from longitudinal analysis of drug use patterns over time? This analysis combined data from five longitudinal studies conducted in California and examined 10-year patterns of heroin, cocaine, methamphetamine (meth), marijuana, and alcohol use for primary users of heroin (n=629), cocaine (n=694), and meth (n=474). Results suggest relatively low levels of use of non-primary heroin, cocaine, and meth, but moderate levels of alcohol and marijuana use. Growth models showed declining primary drug levels for heroin and meth users and relatively consistent levels over 10 years for cocaine users, while levels of non-primary drugs remained at consistently low levels or declined in tandem with the primary drug. Results indicate that group descriptions of primary heroin, cocaine, or meth use trajectories over time may present valid information about drug use patterns in general.