Decades of research have demonstrated a strong link between trauma, PTSD and Substance Use Disorders (SUDs) in general [1,2,3,4] and cannabis use disorders in particular [5]. In one large sample of veterans seeking treatment for PTSD, 14.6% of participants reported using cannabis in the previous six months [6], and 14% of individuals with lifetime PTSD reported past-year cannabis use [7]. In a nationally representative adult sample, lifetime trauma was associated with greater likelihood of lifetime cannabis use, and results showed a graduated relationship between the co-occurrence of the two, with less severe cannabis use being related to less severe trauma exposure [8]. A number of etiological models have been proposed to explain the co-occurrence of PTSD and substance use, including the “self-medication” hypothesis, “high risk” hypothesis, “susceptibility” hypothesis and “shared vulnerability” models [9,10,11,12,13,14]. The self-medication hypothesis posits that individuals with PTSD use substances as a way to manage painful affect states [15,16]. Indeed, a substantial portion of individuals with anxiety and/or PTSD reports symptom relief as a goal of their substance use [17,18,19]. While the self-medication hypothesis has received the most clinical attention and empirical support, several other hypotheses may account for the complex relationships between PTSD and SUD. The high risk hypothesis posits that a substance-using lifestyle (e.g., illegal procurement of drugs) places the individual at high risk for exposure to traumatic events and subsequent development of PTSD [12,14]. Relatedly, the susceptibility hypothesis proposes that chronic substance use impairs one’s neurobiological system, which enhances the likelihood of developing PTSD after trauma exposure [9,11]. Shared vulnerability models implicate common cognitive, affective and neurobiological factors (e.g., attentional bias, emotion regulation difficulties and dysfunction in the hypothalamic-pituitary-adrenal (HPA) axis) in the development of both PTSD and SUD and their associations [20]. For example, previous research shows that adults with either PTSD or SUDs exhibit deficits in emotion processing and self-regulation [21,22].

Cannabis is often used concurrently with other substances, with individuals who used cannabis in the previous six months being more likely than those who do not use cannabis to smoke cigarettes (33.9% vs. 61.9%), engage in hazardous alcohol use (8.89% vs. 11.54%) and other drug use (4.1% vs. 23.8%) [6]. In a sample of adolescents, ongoing regular cannabis use was found to predict the maintenance of other drug use and to be associated with reduced rates of cessation for high-risk alcohol use and use of all other substances, with the exception of cocaine [23]. The latter exception was due to lower prevalence and sporadic use of cocaine compared to other drugs in this sample. A recent prospective study of 34,653 participants found that use of cannabis at baseline was a significant predictor of the development of a SUD three years later, even after adjusting for other sociodemographic factors known to contribute to SUDs [24]. Furthermore, use of multiple substances, often termed polysubstance use, has been associated with elevated risk of psychiatric and physical health problems, an increased risk of dependence and overdose and poor treatment outcomes [25]. Interestingly, Swift et al. found that although overall as cannabis users age, prevalence of cannabis use declines, a greater proportion of users become regular users, and the prevalence of other illicit drug use increases and progresses [23].

Given the widespread utilization of cannabis among those with PTSD, paired with reductions in the perception of the harmfulness of cannabis, investigations have now begun to explore whether the utilization of cannabis prior to, during or after treatment is associated with beneficial or adverse treatment outcomes among those with co-occurring psychiatric and other substance use disorders. Several studies suggest that cannabis use is associated with PTSD symptom reduction and improved coping [26,27]. In a study of coping strategies among veterans experiencing PTSD, a subgroup of participants reported preferring cannabis to other types of substances and experiencing benefits from cannabis use, including reductions in anxiety and depression and prevention of intrusive thoughts and memories [28]. A recent literature review on military veterans with PTSD identified several small studies showing that cannabis and cannabinoid use were associated with reductions in PTSD symptoms [26]. Evidence suggests the psychoactive properties of cannabis may be helpful in reducing anxiety, extinguishing fears and aversive memories, improving depression and enhancing sleep [28,29]. However, chronic cannabis use has been shown to impair fear extinction in preclinical models relevant to PTSD [30,31,32].

The benefits of cannabis use with respect to other drugs of abuse are less clear. Several studies have reported no association between cannabis use and adverse treatment outcomes among opioid- and cocaine-dependent patients [33,34,35,36]. For example, a study of opioid-dependent youth who received psychosocial treatments plus buprenorphine-naloxone found that cannabis use prior to treatment onset and concurrent cannabis use were not associated with opioid use over 12 weeks of treatment [35]. The authors theorized that cannabis may not have had an impact on outcomes because the buprenorphine-naloxone treatment may have adequately addressed many of the symptoms that individuals are likely to use cannabis to relieve (e.g., dysphoria and opioid withdrawal symptoms) [35]. Likewise, a retrospective chart review of individuals during the early phase of outpatient treatment found that cannabis use was associated with fewer days of opioid use and did not negatively impact the methadone induction process during methadone maintenance treatment [36]. Similar results have also been found among cocaine-dependent samples. In a sample of cocaine-dependent individuals seeking treatment, no significant adverse relationships were found between baseline and during treatment cannabis use and treatment retention and cocaine abstinence, suggesting that cannabis use and cocaine use were functionally independent [37]. Given the lack of impairment or adverse outcomes associated with cannabis use, these findings provide support for a harm reduction approach with regard to concurrent cannabis use.

However, other studies suggest that cannabis use either before entering or after being discharged from treatment may be associated with adverse treatment outcomes among opioid and cocaine-dependent patients [38,39,40]. For example, a prospective study of heroin-abstinent patients found that cannabis use was a significant predictor of heroin use resumption [40]. A secondary analysis of two completed clinical trials of treatment for cocaine dependence found an effect of baseline marijuana use on response to treatment for cocaine dependence; specifically, more days of marijuana use at baseline predicted lower treatment effectiveness (cocaine-negative urines) among those receiving levodopa/carbidopa, but not in the placebo condition [39]. The authors speculated that higher baseline marijuana use may be a proxy for cocaine severity. The counter argument that concurrent marijuana use can lead to negative outcomes on other substances proposes that cannabis use may activate the reward systems in the brain associated with alcohol, opioids and cocaine, leading to use or reinstatement of these other substances. These studies further suggest that cannabis use may not be as harmless as perceived and, thus, clinicians should actively assess for and incorporate cannabis use treatment in their client’s aftercare plan.

Despite mixed findings, the majority of studies reported were conducted with predominantly young, male, Caucasian or veteran samples receiving primarily medication-assisted treatments; thus, it remains unclear whether the findings are generalizable to other subgroups of populations. Moreover, none of the studies to date have examined the impact of cannabis use on treatment outcomes in a dually-diagnosed sample with co-occurring PTSD and substance use disorders. Additionally, very few studies have examined the effect of cannabis use on PTSD or primary substance use severity during treatment. The present study thus sought to extend prior findings by examining whether cannabis use at baseline and weekly during treatment had an impact on in-treatment and end-of-treatment PTSD symptom severity and primary substance use among a treatment-seeking dually-diagnosed population. Given the extant literature on the link between cannabis use and PTSD symptoms, we hypothesized that baseline and in-treatment cannabis use would be significantly associated with in-treatment and end-of-treatment PTSD symptom severity. Given the mixed findings in the literature on the impact of cannabis use on other substance use outcomes and the limited data on the effects of weekly cannabis use on in-treatment SUD symptomatology, we examined these associations in an exploratory fashion.