Our study presents the first data on differences in the patterns of consumption of illicit drugs, risky sexual behaviours, and syphilis exposure in crack users in Central Brazil. We found no differences in sociodemographic characteristics between genders for crack cocaine users in Central Brazil. However, as also observed by Bertoni et al., [7], a large difference was found in the pattern of drug use and sexual risk behaviours.

The median age at first illicit drug use by our participants was 16 years (IQR: 6 years). Furthermore, men first used illicit drugs at an earlier age than women (16 years vs. 18 years, respectively; p < 0.001). Other studies have also shown that women start using drugs at a later age than men [16–19]. Gender-related differences in age at onset of drug use are possibly due to men having increased access to illicit substances and a lesser perception of risks related to substance use compared with women [16, 20].

Our findings have some implications for public health. First, the initiation of drug use during early adolescence has been associated with increased dependence and drug abuse [21, 22] and an increased risk of STI acquisition, including HIV [23]. Thus, drug use at an early age can contribute to the overall burden of substance-related illnesses. Furthermore, the results of this study suggest that men have opportunity to access illicit drugs at an earlier age than women. Therefore, interventions and actions to prevent drug use should be intensified for adolescent males.

In the context of crack addiction, poly-drug consumption seems to be a common behaviour [7, 11, 24, 25]. In fact, the majority of our participants reported this behaviour, although it seems to be more frequent among males. A previous investigation of 160 crack users from two large cities in Southern and North-eastern Brazil showed a higher proportion of alcohol (55.7% vs. 25.7%), marijuana (71.5% vs. 51.4%), and intranasal cocaine (45.8% vs. 11.4%) use among men compared with women [7]. These behaviours (except for alcohol use) were also found in our participants from Central Brazil. In addition, the CHAID decision tree model has also demonstrated marijuana consumption as a variable that differentiates the genders. The simultaneous consumption of substances is a risk factor for chemical dependence, the development of psychological disorders (e.g., depression and anxiety), and physical illnesses [26, 27]. Thus, our findings suggest that these related problems of substance use are more likely to occur in men than women.

Studies in humans and animal models have shown that females are more vulnerable to heavy drug use and recidivism compared with males [20, 28]. The oestrogen hormone is believed to play a role in the abuse and heavy consumption of illicit drugs by women [29]. Our findings contribute to this assumption. Compulsive drug consumption was greater among women, with 70.6% of women and 52.0% of men reporting daily crack consumption, results that were supported by the CHAID analysis.

Smoking crack mixed with tobacco and/or marijuana in the form of a cigarette was frequently reported by our participants (~40%); however, this behaviour was more frequent among men in the bivariate analysis (p < 0.05). In qualitative studies using in-depth interviews with crack users, the concurrent use of crack and marijuana seems to diminish the undesirable effects of crack (e.g., cravings), and consequently the risk behaviours and violence related to its consumption. Nevertheless, clinical investigations are necessary to elucidate the real role of this strategy in reducing the harm associated with crack use [30, 31].

Unprotected sexual intercourse was the most frequently reported risky sexual behaviour among our participants of either gender. Unprotected sexual intercourse with a steady partner, unprotected anal intercourse, and unprotected sexual intercourse with casual partners were also reported. These behaviours were also found in a previous study conducted among drug users in the United States of America, although these authors found a higher proportion of unprotected sexual intercourse with steady partners occurring among female drug users compared with males (82.4% vs 75.4%; p = 0.016) [12]. In general, long-term relationships create a sense of security and trust that precludes and/or reduces condom use. A previous study investigated intimacy and its role in relationship management among 73 HIV drug users and their partners. According to this investigation, in the context of HIV, unprotected sex between steady partners strengthens the relationship [32].

The exchange of sex for money and/or drugs and sexual violence were more frequently reported among females than males. In addition, the CHAID model showed that these key sexual risk variables differentiated the genders, ratifying the greater social vulnerability of female crack users compared with males [25, 33–35]. Thus, in addition to intrinsic biological characteristics, these factors may increase the susceptibility of women to STIs, which may contribute to the large disparity of syphilis prevalence between genders found in our investigation. In fact, syphilis prevalence was three times greater in women compared to men in our study.

This study has some limitations. Firstly, all information regarding patterns of illicit drug use and risk behaviours was based on self-reports, which may underestimate the prevalence of certain risk behaviours (e.g., exchanging sex for money and/or drugs). Thus, the data may be subject to memory bias and reported according to social desirability. Secondly, the non-probabilistic sample and the inclusion of subjects from only institutions for the treatment of chemical dependence may not allow the results to be generalised to crack users from other geographical locations. However, the sociodemographic characteristics of our participants were similar to those of other crack users in Brazil, suggesting external validity [5]. Thirdly, the sample of women was relatively small, which may have reduced the analytical power of the study.