Preclinical Evidence for Sex-Dependent Effects

Preclinical models/markers of drug abuse

Several animal studies suggest that females may be more sensitive than males to the reinforcing and discriminative effects of cannabinoids. First, female rats of two strains acquired stable drug self-administration faster, self-administered more of the synthetic cannabinoid agonist WIN55,212-2 during the maintenance phase, took longer to extinguish responding when vehicle was substituted for WIN55,212-2, and showed greater drug- and cue-primed reinstatement than males (Fattore et al, 2007, 2010). Female rats also learned to discriminate THC from vehicle at a lower dose than males (1 vs 3 mg/kg), and acquired the discrimination significantly faster (Wiley et al, 2017). THC discrimination was also acquired in ~30% fewer sessions in female compared to male mice (Wiley et al, 2011). The only study comparing cannabinoid place preference/aversion between the sexes reported no significant THC effect in either sex (Hempel et al, 2017), although the highest dose tested, 6 mg/kg, reduced time spent on the THC-paired side by ~30% in male but not female rats. Thus, the limited number of animal studies published to date suggests that female rodents are more sensitive than males to the reinforcing and discriminative stimulus effects of cannabinoids.

Sex-dependent tolerance and dependence

Several studies have examined sex differences in the development of tolerance to THC in adult female rats compared to males. In two studies comparing tolerance development to the ‘tetrad’ of cannabinoid effects (antinociception, locomotor suppression, catalepsy, and hypothermia) in male vs female, adolescent vs adult rats (Wiley et al, 2007; Wiley and Burston, 2014) or in adolescents bred in-house vs shipped (Wiley and Evans, 2009), robust tolerance developed to all four ‘tetrad’ effects and no sex differences in tolerance development were observed. However, the high-dose (10 mg/kg), 10-day chronic THC regimen led to nearly complete flattening of most dose-effect curves in all groups, so the magnitude of tolerance development was difficult to distinguish among groups. Tolerance to THC’s cataleptic effect did appear to be greater in adolescent females than males in one of these studies, although group sample sizes were relatively small and the sex difference was not statistically significant (Wiley and Burston, 2014). In a subsequent study using doses of THC lower than 10 mg/kg to induce tolerance, female rats developed significantly more antinociceptive tolerance than males, measured as a greater decrease in THC potency in females than males (Wakley et al, 2014). Similar sex differences were reported in a later tolerance study from the same lab, using the same acute pain tests (Wakley et al, 2015). Notably, in the latter two studies the tolerance-inducing dose of THC was sex-specific (~30–40% lower in females than males) to adjust for greater THC potency in females before tolerance induction. Thus, even when treated with lower doses than males, females developed greater tolerance than males. In contrast, another study in adult rats showed that when 10 mg/kg THC was administered daily, more rapid tolerance occurred in males compared to females responding on a repeated acquisition and performance operant task (Weed et al, 2016). Finally, another study in which a low dose of THC (2 mg/kg) was given chronically to juvenile (PND22–40) or late adolescent rats (PND41–60), no tolerance development was observed in either sex (Harte and Dow-Edwards, 2010). In the only study examining the effects of chronic THC in male vs female mice, several days of THC treatment resulted in significantly altered locomotor activity only in females: tolerance developed to the stimulant effects of a low THC dose (1 mg/kg) and sensitization developed to the stimulant effects of a high dose (30 mg/kg; Wiley, 2003). In summary, the few studies that have examined sex differences in the development of tolerance to cannabinoids suggest that females develop tolerance more readily than males to the antinociceptive effects of THC, but there are very few studies of tolerance development to cannabinoid effects other than antinociception. Additionally, cannabinoid tolerance is known to be dose-dependent and to develop at different rates for different effects (Gonzalez et al, 2005), so in the future it will be important to conduct sex comparisons of cannabinoid tolerance to a variety of effects using a variety of dose ranges and dosing frequencies.

In regard to cannabinoid dependence, several studies report sex differences in animals. In one study, a high dose of THC (30 mg/kg) was administered twice-daily for 6.5 days to adult female and male rats, and then withdrawal was precipitated with the CB1 antagonist rimonabant (10 mg/kg). The only significant sex difference among the many withdrawal symptoms assessed was that females showed more retropulsion (walking backwards) than males did (Marusich et al, 2014). A second study conducted in adult rats also used rimonabant (1.0 mg/kg)-precipitated withdrawal: after 40 days of 10 mg/kg/day THC treatment, females showed more disrupted performance (increased errors, decreased response rate) than males in the performance component of a repeated acquisition, 3-response sequence task (Weed et al, 2016). A third study examined spontaneous withdrawal symptoms after termination of chronic THC treatment of adolescent rats. Rats were given either 2, 7.5, or 15 mg/kg daily from PND35–41, and then tested on the elevated plus-maze on PND 42, 44, and 56. Early in withdrawal, males that had been treated with the higher doses of THC showed evidence of decreased anxiety whereas females showed evidence of increased anxiety (Harte-Hargrove and Dow-Edwards, 2012). Overall, limited evidence suggests that female rats may develop greater tolerance and greater cannabinoid dependence than males, although results may be end point-specific, and may also depend on the dose and duration of the chronic cannabinoid treatment.

Clinical Evidence for Sex-Dependent Effects

Epidemiology studies have consistently demonstrated that men have a higher rate of problematic cannabis use marked by increased chronicity of use (Preston, 2006), longer episodes of CUD, and greater cannabis use as measured by number of cannabis cigarettes smoked per day (Khan et al, 2013). Additionally, prevalence rates of CUD are higher among males relative to females during adolescence (Hayatbakhsh et al, 2009, and see Farmer et al, 2015: lifetime prevalence rates of DSM-IV cannabis abuse or dependence from 16 to 30 years of age was 22.5% of the male population vs 16.4% of the female population), and in adulthood (Haberstick et al, 2014: lifetime prevalence of DSM-IV cannabis dependence diagnosis in men was 10.9 vs 6.1% in women). The increase in risk observed among men relative to women may not be due to biological underpinnings, but rather due to social factors that limit exposure to cannabis and decrease the likelihood of cannabis initiation and use in females, including greater perception of risk, decreased cannabis use among peers, and greater childcare responsibilities. However, epidemiological studies are showing that sex differences in cannabis use are shrinking, suggesting that societal constraints on cannabis use among females are abating. An analysis of data from the National Youth Risk Behavior Survey, a large, biennial, school-based survey, demonstrated that while boys consistently show higher prevalence rates than girls for lifetime and current cannabis use, the gap decreased over a 14-year period between 1999 and 2013, with a difference of 7.6 percentage points between boys and girls in 1999 for prevalence of lifetime cannabis use vs a difference of 2.9 percentage points in 2013 (Johnson et al, 2015); this shrinking gender gap has also been observed at the national level among adolescent and adult respondents (Substance Abuse and Mental Health Services Administration, 2014a,). Furthermore, findings from the annual National Survey on Drug Use and Health (NSDUH) from 2002 through 2014 show that past-month cannabis use increased among pregnant women from 2.37% to 3.85%. Among non-pregnant women, past month use increased by close to 4% in women 18–25 years of age, and close to 3% in women overall (18–44 years of age; Brown et al, 2017). Given the narrowing gap in cannabis use among adolescent and adult males vs females, identifying sex-dependent effects of cannabis associated with the development of CUD and probing the biological basis for these differences is a public health imperative.

Abuse-related subjective effects

Sex differences in response to the acute effects associated with abuse liability of cannabis and cannabinoids have been examined using double-blind, placebo-controlled laboratory procedures. End points that have been assessed include subjective drug ratings related to positive drug effects and cue-induced craving. Similar to studies investigating sex differences in the therapeutic effects of cannabinoids, sex differences related to abuse liability were mostly analyzed as an exploratory aim or by pooling data from several studies in order to obtain sufficient statistical power to detect a significant difference between men and women. A recent laboratory study compared the discriminative stimulus and subjective effects of oral THC in male and female non-treatment-seeking cannabis users. While men and women did not appear to be differentially sensitive to the discriminative stimulus effects of oral THC, a dose-dependent sex difference emerged with respect to THC’s subjective effects that are indicative of abuse liability. Women exhibited greater sensitivity to the lowest THC dose tested (5 mg) and reported higher ratings related to abuse liability, including ratings of drug liking, desire to take the dose again, and good drug effect. In contrast, men reported higher subjective ratings of drug liking than women for the higher THC dose (15 mg). The sample used for this study consisted of men and women who did not differ in cannabis use, with average use reported to be 4.5 days per week and about 3 cannabis cigarettes/day (Fogel et al, 2017). An earlier study investigating the effects of higher oral THC doses (20 and 40 mg) in male and female cannabis smokers failed to find sex-dependent effects with regard to subjective ratings. However, differences emerged when assessing these effects in non-cannabis users: men were more sensitive than women to the intoxicating effects (ie, ratings of ‘High’) of oral THC (2.5–10 mg) (Haney, 2007).

Studies with smoked cannabis reveal mixed effects. One study that investigated the subjective ratings of smoked cannabis included participants whose smoking frequency ranged from once per month to three times per week. Women were reported to have a slower latency to detect cannabis’s psychoactive effects and a shorter duration of its effects compared to men, suggesting that men were more sensitive to cannabis’s subjective effects (Penetar et al, 2005). This study did not specify if men and women differed in current cannabis use. Given the wide range of cannabis use frequency among participants in this study (one time per month to three times a week), differences in current cannabis use between men and women may have influenced the findings. A second study assessing sex-dependent subjective effects of smoked cannabis matched men and women according to current cannabis use (Cooper and Haney, 2014). Among daily cannabis smokers, females were more sensitive than males to subjective effects related to abuse liability, including ratings of drug liking and willingness to take the drug again. However, males and females did not differ in ratings of intoxication (‘High’). These findings indicate that under double-blind, placebo-controlled procedures, female daily cannabis smokers are more sensitive than their male counterparts to the subjective experience that is associated with cannabis abuse liability. Although subjective effects provide a measure of abuse potential, drug self-administration is the best measure of abuse liability because it best reflects drug-taking in the natural ecology. To date, no studies have assessed possible differences in cannabis or cannabinoid self-administration between men and women.

Together, these findings suggest that sex differences in THC-induced subjective effects depend on dose, route of administration (oral vs smoked), and population (cannabis users vs non-users). These findings are directly relevant to the population using cannabis for medical indications, which varies in previous and current exposure to cannabis and cannabis-derived products. In addition, these findings highlight the differences across studies that can emerge as a function of route of administration, a variable that should be addressed in future studies, particularly given the increasing popularity of edibles (Cuttler et al, 2016).

Cue-induced craving

Cannabis craving as a measure of addiction severity has been studied as a function of participant sex. Men and women who smoked similar amounts of cannabis were presented with tactile, visual, and olfactory cannabis-related cues and were asked to rate their craving for, urge, and desire to smoke cannabis. No sex differences were detected in baseline craving. While craving for, urge, and desire to smoke cannabis increased in response to the cues, differences between men and women were not detected (Lundahl and Johanson, 2011). A second study probed the potential utility of oral THC (10 and 20 mg) for treating CUD in daily cannabis smoking men and women by determining its effects on cannabis cue-elicited craving (Lundahl and Greenwald, 2015). In this study, cannabis cues increased ratings associated with compulsivity, or inability to control cannabis use, anxiety, and urge to smoke cannabis relative to a neutral cue. Oral THC decreased cue-elicited craving and anxiety, an effect that did not differ between men and women. However, sex-dependent effects were observed for ratings of compulsivity, with women exhibiting significant decreases on this measure after receiving either dose of oral THC, whereas oral THC failed to attenuate this rating in men (Lundahl and Greenwald, 2015). These data suggest that THC differentially attenuates aspects of subjective responses related to continued cannabis use between men and women, and have implications for the use of THC as a potential pharmacotherapy for CUD.

Withdrawal

Although women and men report similar patterns of cannabis use, women seeking treatment for problematic use score higher on a measure of severity of drug dependence (Copeland et al, 2001). One indication of severity of drug dependence is the presence and severity of withdrawal symptoms after cessation of cannabis use. Earlier studies examining withdrawal symptoms among non-treatment-seeking male and female cannabis users found that men and women reported different symptoms: women reported nausea, whereas men endorsed goose-bumps and cannabis craving (Agrawal et al, 2008; Copersino et al, 2010). A later survey similarly assessed sex differences in self-reported withdrawal symptoms and also found that more women reported nausea and anxiety, whereas men more frequently reported sleep disruptions (Cuttler et al, 2016). A recent study sought to study incidence and severity of withdrawal symptoms as a function of sex during a cannabis quit attempt in a population of treatment-seeking cannabis users (Herrmann et al, 2015). This study carefully controlled for cannabis dependence inclusion criteria, excluded other drug dependencies (eg, alcohol) and characterized participants based on amount of cannabis use in the previous 90 days, ensuring that men and women were matched for current cannabis use. Women reported more withdrawal symptoms relative to men and had significantly higher scores on symptoms related to mood (including increased restlessness, irritability) and gastrointestinal symptoms (nausea and stomach pain). Women also reported greater symptom severity related to mood symptoms. A later study of treatment-seeking cannabis users similarly found that women endorsed more withdrawal symptoms and negative impact of withdrawal symptoms relative to men who were matched for past-month cannabis use; sex differences in the severity of specific symptoms were found for hot flashes, headache, mood swings, irritability, gastrointestinal symptoms, and sleep disturbances (Sherman et al, 2017). While men and women had similar patterns of cannabis use, women were more likely to have psychiatric diagnoses and reported lower quality of life compared to men, which may have contributed to differences in self-reported incidence and severity of withdrawal symptoms.

Clinical trials for CUD

Clinical trials investigating potential pharmacotherapies to treat CUD do not typically assess outcomes according to sex (recent studies include Allsop et al, 2014; Levin et al, 2016). However, recent studies investigated the potential for sex-dependent treatment effects with the selective serotonin receptor inhibitor and partial 5-HT1A agonists, vilazodone and buspirone, for cannabis dependence (McRae-Clark et al, 2015, 2016). While these pharmacotherapies were not effective in reducing cannabis use relative to placebo for either sex, the authors found that women had worse outcomes relative to men in relation to cannabis craving and in difficulty achieving abstinence. These findings agree with earlier reports demonstrating that women report greater withdrawal symptom severity relative to men.

Given that severity of withdrawal is hypothesized to contribute to ongoing use and relapse to use in a treatment setting (Budney et al, 2008; Haney et al, 2013), these studies suggest that women may experience greater difficulty than men achieving and maintaining abstinence from cannabis use. Understanding how men and women differ in regard to incidence and severity of withdrawal symptoms during abstinence can help to guide research into sex-specific pharmacotherapies for cannabis dependence, increasing treatment success for both men and women. A limitation of these studies is that assessments were based on retrospective self-report measures, requiring participants to recall from memory the incidence and severity of their withdrawal symptoms. In addition, it is not clear whether cannabis exposure immediately prior to the quit attempt was comparable among men and women. Future studies should examine withdrawal symptoms according to both self-report and clinician-based assessments in real time, optimally after participants have been exposed to a known quantity and strength of cannabis. Accurate assessment of pharmacotherapies to target sex-specific withdrawal symptoms can be best examined under these circumstances.

Other End points related to cannabis addiction severity

Differences among men and women in neural correlates associated with severity of cannabis addiction were recently explored in a positron emission tomography (PET) study that measured brain glucose metabolism utilizing the radiotracer [18F]deoxyglucose. Differences in brain glucose metabolism were compared in healthy, non-cannabis-using controls relative to daily cannabis users who met DSM-IV criteria for cannabis abuse or dependence in response to placebo administration and a methylphenidate challenge (Wiers et al, 2016). Under placebo conditions, cannabis users exhibited lower glucose metabolism relative to healthy controls in the frontal cortex and anterior cingulate. In response to the methylphenidate challenge, cannabis users exhibited blunted regional metabolism compared to healthy controls; moreover, regional increases in glucose metabolism in response to methylphenidate were negatively correlated with addiction severity. In regard to sex differences, the differences between healthy controls and cannabis users in both placebo and methylphenidate-induced changes in metabolism were driven by the female cannabis users. That is, female cannabis users exhibited lower glucose metabolism after placebo administration relative to female healthy controls, an effect that was not observed when comparing cannabis-using men to non-cannabis-using men. Furthermore, increases in methylphenidate-induced glucose metabolism were apparent in the female healthy controls but not in the female cannabis users. The authors conclude that women who use cannabis may be more susceptible than men to the adverse neurobiological effects of cannabis, including negative emotionality and severity of addiction (Wiers et al, 2016).

The above findings demonstrate that men and women differ in End points associated with cannabis’s abuse liability, withdrawal from cannabis, and neurobiological End points associated with cannabis addiction severity. These effects were assessed in response to smoked cannabis and oral THC in populations of non-cannabis users, light users, and heavy users. Multiple studies report that cannabis-dependent women experience more difficulty than cannabis-dependent men with withdrawal symptoms, a finding that is consistent with results showing enhanced negative effects of cannabis in women who are daily smokers, including increases in subjective effects that reflect abuse liability, and deficits in neurobiological markers associated with addiction severity. Given the differences observed across studies as a function of cannabis exposure, these findings highlight the need to match women and men according to current cannabis use in order to control for the potential effects of tolerance that may alter adverse effects of cannabis, withdrawal, cue-induced craving, and investigation of neural markers of addiction severity. A significant gap in our knowledge regarding differences between men and women in relation to CUD is self-administration during non-abstinent and abstinent conditions, and a systematic assessment of withdrawal symptoms as a function of sex while controlling for cannabis exposure preceding cessation of use, as well as withdrawal assessments that include clinician-based ratings and objective measures.