In a combined model examining alcohol, cigarettes, and marijuana across initiation and intensity of use, we found that number of sexual partners steadily increased as substance use intensified from never use to experimental/new user to heavy use across all substances for both male and female high school seniors. This is the first known study to use a nationally representative sample and show such a stepwise and seemingly uniform increase in likelihood for number of sexual partners corresponding with the severity of substance use across alcohol, cigarettes, and marijuana use. These findings persisted even while simultaneously controlling for age of substance use initiation as well as race/ethnicity and age of participant. For marijuana users only, odds of higher number of sexual partners also steadily increased as age of initiation decreased. The overall findings of our study further suggest that severity of alcohol and cigarette use is a better indicator of higher number of sexual partners than age of initiation. In contrast, intensity and age of initiation are both important in their associations with number of sexual partners when examining marijuana use.
Specifically, alcohol use was associated with having more sexual partners across all outcome categories (i.e., 1, 2–5, 6 or more sexual partners versus no sexual partners) for both males and females. In general, the risk for higher number of sexual partners increased as alcohol use intensified from experimental to moderate to heavy use. Age of initiation did not appear to be associated with increasing risk of having more sexual partners. The association between alcohol use intensity and having more sexual partners was greater for females than males.
For males, cigarette use was associated with having more sexual partners (2–5 partners and 6 or more partners versus no partners) when it occurred regularly (i.e., at least once a day for 1 month). In fact, the risk was about twofold when daily cigarette smokers initiated cigarette use at age 13 or above but was about threefold when initiated at age 12 or younger. For females, cigarette use was associated with having more sexual partners irrespective of intensity of use; however, the risk was greater for daily users, especially when comparing 6 or more sexual partners versus those who had not yet had sexual intercourse.
Marijuana use was associated with having more sexual partners across all outcome categories (i.e., 1, 2–5, 6 or more sexual partners versus no partners) for females and for 2–5 and 6 or more sexual partners (versus no partners) for males. Like alcohol use, the risk for higher number of sexual partners (i.e., more than 1 partner) generally increased as marijuana use intensified from experimental to moderate to heavy use. Age of marijuana initiation appeared to be associated with having more sexual partners (i.e., more than 1 partner) for male users across all intensity levels (i.e., experimental, moderate, and heavy use) and females who became heavy marijuana users. In addition, heavy marijuana use appeared to have a stronger association with increasing risk of having more sexual partners for females than males.
While our findings correspond with past reports that link substance use with risky sexual behaviors, this study makes significant contributions to the existing literature on this topic. Specifically, the large sample size in the present study enabled us to improve the measurement of substance use and its association with number of sexual partners. Since risk for higher number of sexual partners occurred across multiple substances, these results do not support alcohol-induced disinhibitions which has been alleged to cause risky sexual behaviors via impaired decision making [12
]. While our findings do not necessarily contradict associations between alcohol-induced disinhibitions and risky sexual behaviors, we assert that the association of high number of sexual partners and alcohol use found in past reports could likewise be explained by risk behaviors that cross multiple domains which also drive cigarette smoking and marijuana use. We further add to the existing literature the important finding that risk behavior associations between substance use and number of sexual partners appear to trend in a stepwise direction. That is, the likelihood for having more sexual partners steadily increased as substance use behaviors became more severe. Lastly, an association between age of initiation and number of sexual partners was found for marijuana use only but not for alcohol or cigarette initiation. This finding might be due to unique environmental and/or biological vulnerability factors that differ with each of the substances initiated and their influence on risky sexual behaviors. Our findings might also reflect the tendency for alcohol and cigarette initiation to be a common occurrence during adolescence while marijuana initiation (especially when it occurs at an earlier age) is not and thus indicates a proneness to deviant behaviors.
Thus, the implications of our findings are relevant. Specifically, the varying levels of substance use behaviors examined in our study can provide warning signs to parents and/or a variety of professionals such as school counselors, nurses and physicians who are in contact with adolescents and can potentially measure risk for pregnancy and STD transmission and deliver prevention messages. Moreover, we provide further justification for interventions that cross multiple domains of risk (i.e., substance use and risky sexual behaviors) and target substance users, especially those who are heavier users.
The findings of this study were limited by several factors. The YRBS is cross-sectional, preventing the determination of causal relationships, and the survey does not provide information on potential explanatory variables such as socioeconomic status, social class, family structure, and/or relationships with friends or intimate partners that are needed for more in-depth analyses. In addition, YRBS is a school-based survey which does not include data from high school dropouts or adolescents schooled at home. We also rely on participants’ self-report for all of the data which contain some unknown level of reporting error. Substances, including cocaine, heroin, and methamphetamines, were omitted from our analyses due to limited use.
Nevertheless, our findings demonstrated a clear pattern of increased odds for higher number of sexual partners associated with increase of substance use. Importantly, the same was not found for age of substance use, with the exception of age of onset for marijuana use. While we recognize that age of substance use initiation is an important indicator of adolescent risk behaviors, in general, our findings suggest that severity of substance use is more strongly associated with number of sexual partners among adolescent high school seniors. In summary, our study improves the measurement of substance use and enables prevention efforts to more accurately assess and target adolescents who are at high risk for pregnancy and STD transmission and need preventive intervention.