Simultaneous polysubstance use (SPU) is a common phenomenon, yet little is known about how various substances are used with one another. In the present study 149 drug-using university students completed structured interviews about their use of various substances. For each substance ever used, participants provided details about the type, order and amount of all substances co-administered during its most recent administration. Alcohol, tobacco and cannabis were frequently co-administered with each other and with all other substances. Chi-squared tests revealed that when alcohol was used in combination with any of cannabis, psilocybin, MDMA, cocaine, amphetamine, methylphenidate (ps < 0.01) or LSD (p < 0.05) its initial use preceded the administration of the other substance. Paired samples t-tests revealed that when alcohol was used with cocaine (p < 0.01) or methylphenidate (p < 0.05) it was ingested in greater quantities than when used in their absence. Patterns of cannabis use were not systematically related to other substances administered. Finally, using one-sample t-tests, tobacco use was demonstrated to be increased relative to 'sober' smoking rates when used with alcohol, cannabis, psilocybin, MDMA, cocaine, amphetamine (ps < 0.001), LSD (p < 0.01) or methylphenidate (p < 0.05). Results suggest that many substances are routinely used in a SPU context and that the pattern in which a substance is used may be related to other substances co-administered.

Those who misuse MPH are more likely than their peers to misuse various other substances, and MPH misuse frequently occurs in the context of simultaneous polydrug use. Because the primary supply of inappropriately used MPH appears to be prescribed users, efforts should be directed toward preventing its diversion.

Alcohol and ED co-administration is relatively common among ED users and seems to be associated with increased alcohol ingestion. It is recommended that this matter receive more clinical and research attention.

Poor medication compliance, diversion, and misuse are relatively common and interrelated among adult MPH users. MPH prescriptions should be monitored closely in individuals with histories of illicit substance use.

Such findings raise the possibility that specific alcohol, tobacco and/or cannabis use episodes might directly contribute to the initiation of new substance use. Understanding the role of SPU on occasions of first use might help better identify risk factors for substance use progression and improve intervention efforts.