Few studies have addressed the distinction between concurrent polydrug use (various drugs used on separate occasions) and simultaneous polydrug use (the use of more than 1 drug at the same time). The authors assessed simultaneous polydrug use in a community sample to examine the prevalence of drug combinations, whether simultaneous can be distinguished from concurrent, and the prospective effects of these styles of drug use on subsequent health service utilization, physical symptoms, and psychological distress 4 years later. Marijuana and alcohol were the most common drugs used simultaneously, followed by alcohol and cigarettes. Simultaneous and Concurrent Polydrug Use formed 2 correlated but discriminable constructs. Neither Simultaneous nor Concurrent Polydrug use predicted subsequent Health Service Utilization, Physical Symptoms, or Psychological Distress. Data did reveal unique effects of specific drugs used simultaneously on these outcomes that were larger and more numerous than specific effects of concurrent drug use.