The overall prevalence of lifetime ever use and confirmed current use of at least one drug was 58% and 51%, respectively. Based on self-reported history, participants were likely to use alcohol, marijuana, tobacco products, and khat. Confirmatory tests agreed with reported drug use, as most participants tested positive for alcohol, cotinine, tetrahydrocannabinol, and amphetamine. Predictors of drug use were being a third-born child, having a year of study, receiving pocket money ranging between USD 50 and USD 100, and being sexually active. This was applicable for both self-reported history and confirmed drug use.

Alcohol was the most consumed drug in this study. The high prevalence of alcohol usage concurs with the national report that alcohol is the most used drug in Kenya [3]. Similarly, alcohol has been shown to be the most preferred drug by university students in Kenya [38, 40], while in Ethiopia [34] and Uganda [35], almost half of the students consume alcohol. Outside Africa, the prevalence rate of alcohol consumption among university students ranged from 19.9% to 33% in Iran, Ireland, and the United Kingdom [15, 16, 46].

The rampant use of alcohol by university students can be attributed to the fact that alcohol is considered a gateway drug [47]. Additionally, the college environment could also be playing a role in alcohol consumption. Some of the risky environmental factors include living in a hostel with a higher number of roommates, partying, and environmental cues [48, 49]. The high rate of alcohol use could be due to its legality [50] which makes alcohol easily available, accessible, acceptable, and affordable.

In this study, tobacco products were the second most popular drug used, and students were likely to smoke cigarettes or shisha. This was supported by the participants testing positive for cotinine. In Kenya, a lifetime prevalence rate of tobacco use of 42.8% has been reported among students in tertiary institutions in Eldoret [38], while in Rwanda, a prevalence of 26.1% was reported [22]. Elsewhere, in Sudan and Egypt, tobacco use was most prevalent among university students at 13.7% and 8.9%, respectively [23, 27], while smoked and nonsmoked tobacco were some of the most preferred drugs by university students in India [51].

The present study reports marijuana use by university students. These findings concur with results documented in South Africa as well as Egypt, where 30.9% and 24.3% of students reported lifetime use of Cannabis [27, 36]. In the mid-Atlantic (USA), marijuana was the most commonly used drug among college students in every year of the study [9]. Likewise, across 49 medical colleges in the USA, about 26.2% of the medical students had used marijuana in the past one year [8]. The possession, sale, growing, and distribution of marijuana in Kenya are illegal [52]. Despite this, marijuana is unlawfully grown in Kenya, making it relatively easily available and accessible. Of great concern, especially in learning institutions, are the effects induced by delta-9-tetrahydrocannabinol (THC) on verbal, thinking, and working memory [53].

We report the use of khat in the present study. A previous study reported 14.6% lifetime use in Ethiopia [54]. Findings indicating a high prevalence of khat use are similar to those of a systematic review that reported khat as one of the most consumed substances among university students in Ethiopia [55]. Further, a meta-analysis indicated that the pooled prevalence of current khat use among university students was highest in Saudi Arabia (18.9%), followed by Ethiopia at 13.6%, and 13% in Yemen [31]. Testing positive for amphetamine could be due to chewing khat which is a common practice in Kenya and more so in Mombasa. Khat seems to be an emerging gateway drug.

In the present study, a significant proportion of university students who tested positive were polydrug users. Irrespective of the method used to record data, alcohol, tobacco products, marijuana, and amphetamine or khat were the most preferred combinations. Polydrug use has been previously reported in a Kenyan university, with the main combination being Cannabis, tobacco, and alcohol [40]. In Ethiopia, khat use was strongly and positively associated with alcohol consumption [56]. Concurrent use of Cannabis and tobacco has been previously reported among university students in Spain and Brazil [57, 58].

Polydrug use could be for purposes of enhancing the potency of a drug, reducing its side effects, or alleviating withdrawal symptoms. For example, drinking alcohol immediately after smoking Cannabis intensifies the effects of the latter. This is so because alcohol increases the absorption of THC, thus enhancing plasma THC levels [59]. Polydrug use among university students can also be attributed to the college environment which not only facilitates drug acquisition but also contains environmental cues that trigger substance cravings [48].

In this subpopulation, being a third- or fourth-born child increased the risk of drug use, while being a second-born was protective. These results corroborate the findings of a study carried out in the Netherlands among adolescents, whereby acute alcohol intoxication was more likely to occur in participants who had an older sibling. Thus, being the only child or the first born child was considered a moderating factor for acute alcohol intoxication [60]. Similarly, in Sweden, after-born siblings had a higher likelihood of getting hospitalized for both alcohol and narcotics use [61]. This could be because the latter-born children are likely to be exposed to drugs at younger ages through older siblings.

The year of the study predicted drug use, as participants in the second, third and fourth years and above had an elevated risk of substance consumption. Similar findings were reported in Ethiopia, where being in the third year of study compared to the first year increased the odds of substance use [62]. In Sudan, being in the third year carries a higher risk of drug use than being in the fifth year of study [23]. Findings on the association between the third year of study and drug use are comparable to an eight-year longitudinal analysis in the mid-Atlantic that reported the highest annual peak of marijuana use in year three of the study [9].

This could be attributed to factors like academic-related stress [63], poor social skills, thus increased vulnerability to peer pressure to use drugs [64], limited physical activities [65], and poor quality dating relationships [66]. All the aforementioned factors combined increase vulnerability to stress and depression which in turn heighten drug use as a form of self-medication. Moreover, based on general observations, most universities tend to pay more attention to students in their first year of training than students in subsequent years. This is evidenced by the matriculation ceremony and life skill-related courses that are mandatory in the first year. Additionally, most parents tend to lose touch with their children as they progress into their senior years because they perceive the latter to have matured. Less parental closeness has been reported to be a predictor of substance use [67].

Living within the university hostels or around the university was a moderating factor in this study. On the other hand, students who lived away from the university around town were at the highest risk of drug use. This concurs with the findings of a study at the University of Nairobi in Kenya, whereby students living off campus were more likely to use drugs as compared to their on-campus counterparts [40]. Similar results were reported among university students in the Northeast of the United States, where living off campus posed the highest risk of alcohol use [4]. This could be attributed to the fact that students living off-campus are more likely to reside on the low-income side of town. Poor neighbourhoods act as a haven for drug peddlers and illicit liquor breweries, making drugs easily accessible.

There was a higher risk of drug use among students with a moderate stipend (USD 50 and USD 100) than among those with extremely low income (<USD 50) or very high income (>USD 100). Extreme poverty and superfluity seem to be a moderating factor in this study. These findings differ from the results of a study across 23 public and private USA institutions that reported undergraduates with high socioeconomic status were more likely than peers to use marijuana as well as practice polydrug use [68]. Similarly, among university students in Ethiopia, having a higher monthly income predicted drug use [69]. Among high school students in North Carolina, having a weekly disposable income over USD 50 was a correlate of current hookah use [70]. In Kuwait, students with high family income were more likely to use shisha [71]. This could be attributed to the fact that drug use has a cost implication.

In the present study, drug use was associated with having one or more sexual partners. These findings correspond to the results of a study carried out at Mbarara University in Uganda, where students who reported alcohol use were more likely to have multiple sexual partners [35]. A positive association between higher use of khat as well as tobacco products and multiple sexual partnerships has also been reported in Ethiopia [71]. Outside Africa, university students from nine countries belonging to the Association of Southeast Asian Nations (ASEAN) with multiple partners were significantly more likely to be current tobacco smokers and binge drinkers [72]. Additionally, a systematic review reported an amplified association between alcohol and risky sexual behaviors among university students [73]. This association could be linked to the fact that drugs like alcohol reduce inhibition towards sex, while Cannabis is a vasodilator, thus enhancing sexual libido [74, 75].

This being a cross-sectional study, it is difficult to determine whether the outcome followed exposure in time or whether exposure resulted from the outcome. The strength of the study is anchored in confirmed drug use. This study was able to detect drugs which are currently being consumed using saliva analysis. To the best of our knowledge, this is the first study in Kenya to document validated drug use among university students. Although saliva analysis is not prone to social desirability bias, it only detects drugs used in the past few days, hence the limitation.