Several recent studies suggest that the nonmedical use of prescription drugs is associated with heavy drinking behavior among adolescents and young adults in the United States (e.g., Inciardi et al., 2004; McCabe et al., 2004, 2005a, McCabe et al., b; SAMHSA, 2004a). In a national study of U.S. college students, nonmedical users of prescription stimulants were over six times more likely to report frequent heavy drinking than their peers who did not report nonmedical use of prescription stimulants (McCabe et al., 2005a). In another national study of U.S. college students, nonmedical users of prescription opioids were over four times more likely to report frequent heavy drinking than their peers who did not report nonmedical use of either of these prescription opioids {McCabe et al., 2005b). Finally, a third study surveyed 8th-and 11th-grade students in the Delaware Public Schools and found that the majority of 8th-grade nonmedical users of pain medication (71%) and almost all of the 11th-grade nonmedical users of pain medication (92%) also misused alcohol (Inciardi et al., 2004). Although the nonmedical use of prescription drugs seems to be strongly associated with heavy drinking behavior, little is known about the extent to which this drug-use behavior is concurrent versus simultaneous, and very little research has examined the prevalence, correlates, and consequences associated with these two types of polydrug use.

To date, main studies have not differentiated between concurrent and simultaneous polydrug use (see Schensul et al., 2005). Concurrent polydrug use refers to the use of more than one drug in the same time period (e.g., 12 months) but not necessarily at the same time (Martin et al., 1992, 1993). Simultaneous polydrug use refers to the co-ingestion of different drugs at the same time (e.g., Collins et al., 1998; Earleywine and Newcomb, 1997; Martin et al., 1992). Thus, simultaneous polydrug use is a subset of concurrent polydrug use. Although marijuana and alcohol are the most common drugs used simultaneously, adolescents and young adults have particularly high rates of other types of simultaneous polydrug use (e.g., Collins et al., 1998; Earleywine and Newcomb, 1997).

The factors that were significantly associated with various forms of simultaneous polydrug use among undergraduate students included being male, being white (compared with Asian), and experiencing early initiation of alcohol use. Our results are concordant with Martin and colleagues’ (1992) findings that male gender is predictive of simultaneous polydrug use among college students. Furthermore, the heightened risk for simultaneous polydrug use among early initiators of alcohol augments a wealth of information showing early initiation of alcohol is associated with an increased risk for developing alcohol use disorders (Grant and Dawson, l997).

The findings of the present study have several important implications for prevention and intervention efforts. The present study found that simultaneous polydrug users were more than two times more likely than concurrent polydrug users to report three or more drug use-related problems based on the DAST-10. These findings reinforce the importance of examining individuals who simultaneously co-ingest substances separately from those who report concurrent use of substances. Previous studies that examined only concurrent polydrug use may not accurately reflect the short-term and long-term consequences associated with simultaneous polydrug use. The simultaneous use of alcohol and prescription drugs can be an especially dangerous form of drug misuse because simultaneous polydrug use can have several different effects on the consumer. For instance, using more than one drug at the same time can cause cumulative effects among the drugs (additive effect) or produce a synergistic effect beyond the additive effect of each drug (supra-additive effect). Based on a review of drug-induced fatalities, there is evidence that prescription opioids are much more toxic when they are taken with other drugs that depress the central nervous system, such as alcohol, as compared with when prescription opioids are taken alone (Cone et al., 2004). Collegiate prevention programs that provide drug-specific information and educate about blood alcohol concentration levels and safe-drinking guidelines may not be applicable when students are co-ingesting alcohol and other drugs. Therefore, future collegiate prevention efforts should also educate college students regarding the dangerous drug interactions that may occur when combining alcohol with prescription or illicit drugs.

The present study has several strengths that build on past research examining simultaneous polydrug use. First, the measures used in the present study were consistent with recent recommendations for measuring simultaneous and concurrent polydrug use (Schensul et al., 2005). Most notably, we examined several new specific combinations of simultaneous polydrug use, and we assessed the frequency of simultaneous polydrug use, which previous research has strongly encouraged (e.g., Collins et al., 1998; Schensul et al., 2005). Second, the focus of the present study extended beyond a single class of prescription drugs to four distinct classes of prescription drugs. Furthermore, many previous studies examining simultaneous polydrug use among adolescents and young adults have been limited by the use of general categories of prescription drugs such as “uppers” or “downers” without specific examples of drugs within each prescription drug class (e.g.. amphetamine/dextroamphetamine methylphenidate). Third, the present study focused on simultaneous and concurrent polydrug use involving alcohol and prescription drugs, because college students tend to drink more heavily than their noncollege peers and other age groups (Bachman et al., 1997, 2002; Johnston et al., 2005; O’Malley and Johnston, 2002). Also, the nonmedical use and misuse of prescription drugs has been increasing over the past decade and is higher among young adult drug users (18–24 years of age) relative to other age groups (Johnston et al., 2005; SAMHSA, 2003a SAMHSA, 2004c). Finally, the present study assessed several alcohol-related and drug-related problems, which allowed us to examine a wide range of consequences associated with simultaneous polydrug use.

The current study has some limitations that should be taken into account while considering implications of the findings. First, nonresponse may have introduced potential bias in the present study. Although we can never fully eliminate the possibility of bias introduced through nonresponse, we tried to assess its potential impact by conducting a brief telephone survey of 159 randomly selected nonrespondents. No significant differences in alcohol and other drug use between respondents and nonrespondents were found. Second, the findings of the present study may not generalize to other college and noncollege populations because they were obtained from undergraduate students at a single institution of higher education. Although the present sample resembles the demographic characteristics of students attending 4-year U.S. colleges and universities nationally, previous research has found that rates of alcohol use and nonmedical use of prescription drugs vary across different types of U.S. colleges and universities as well as between young adults attending college versus those not attending college (e.g., Johnston et al., 2005; McCabe et al., 2005a,b). Therefore, future work is needed to examine whether these findings are replicated within other college and noncollege populations. Third, we did not examine the quantity of alcohol and prescription drug consumed on each occasion or other combinations of poly drug use. Furthermore, our investigation likely underestimates the extent of simultaneous use of alcohol and prescription drugs, because we limited our focus to prescription drugs not prescribed to an individual, and thus we did not consider the co-ingestion of alcohol and prescription drugs that were prescribed to an individual.