Cannabis use was highly comorbid with other substance use disorders. Illicit drug usage is 10 times more common in males than females, and there is a huge gap related to treatment for substance use between genders. This could be the reason for the representation of only male patients in our study.

Cannabis use and other substance use disorders

The National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) survey had shown that around half of those with cannabis use in the past year also met diagnostic criteria for alcohol and nicotine use disorders [11]. Among illicit drugs, cocaine, sedatives, stimulants, club drugs, and opioid use disorders were common in those who use cannabis [12].

In our study, alcohol use and nicotine use were seen in 52% and 60%, respectively. Other substance use was seen in a negligible percentage of subjects. Though cannabis along with alcohol and nicotine can act as a gateway drug, availability and affordability also play a significant role in the presence of other substances of abuse. Exposure to other illicit drugs has been low in Andhra Pradesh with the exception of cannabis. This might explain the low incidence of other illicit drug use in our study sample.

Relationship between cannabis use and cognitive functioning

There was a statistically significant difference between the mean MoCA scores of cannabis users and the comparison group (P < 0.001). The majority of the past studies have suggested a significant cognitive decline in cannabis abusers compared to non-abusers and healthy controls [13,14,15,16]. Bassiony et al. in their case-control study on 1682 adolescents found that adolescents who use cannabis frequently had impairment in cognitive functions, and their total MoCA score was less than the controls [17]. Bartholomew et al. reported prospective memory impairments associated with cannabis use in young adults [18]. In India, Wig and Varma reported cognitive disturbances such as poor attention span and memory deficits in bhang users [19]. Mendhiratta et al. in their study reported delayed reaction time, poor concentration, and poor time estimation in long-term cannabis users [20].

We found a statistically significant negative correlation between duration (r = −0.296, P = 0.036) and quantity of cannabis use (r = −0.491, P < 0.001) and cognitive functioning in our study subjects. Studies have proven that cannabis-related cognitive impairment depends upon duration and dosages of use [21]. Adolescents who start using cannabis at an early age generally use a very high amount of cannabis. Cannabis use during critical developmental periods in the still-maturing brain may induce persistent alterations in brain structure and brain function. Studies have also proved that cannabis use can alter the pruning process in adolescents resulting in long-term consequences [22]. Structural imaging studies showed abnormalities in hippocampal volumes and grey matter density in temporal lobes of cannabis users relative to controls [23].

Effect of abstinence on the improvement of cognitive function

Wallace et al. reported improvement in cognitive functions after 2 weeks of monitored abstinence [24]. Rabin et al. [25] in their study found that cannabis-related cognitive impairment is state dependant, and they are reversible with a sustained period of abstinence [26]. Other studies done on chronic cannabis abusers also found significant improvement in cognitive functions after a period of more than 1 month of abstinence [26, 27].

In our study, we reassessed the cognitive functioning after abstinence period of 3 months from cannabis use and found a statistically significant difference in MoCA score (P < 0.001). Though there was a significant improvement in the mean MoCA score in cannabis users after 3 months of abstinence, this score was still lower than the MoCA score of the control group, with a statistical significant difference between these scores (24.08 ± 2.66 vs 28.62 ± 0.85, P < 0.001). This clearly denotes that cannabis use has long-term effects on cognitive functions, and abstinence improves these functions up to some extent only. This can be explained by findings of the previous studies which found that the structural changes induced by chronic cannabis use particularly in the hippocampus and amygdale were persistent even after a prolonged period of abstinence resulting in long-term effects on cognitive functioning [23].

Limitations of the study

The sample of subjects in our study represents an extreme end of the cannabis use spectrum, i.e. patients using higher quantities of cannabis for longer duration. This may not represent the cannabis users in the community. The subjects in the case group are also taking other substances of abuse which can also have effect on their cognitive functions and their performance on MoCA test. Sample size calculation was not done, and the sample size was small. So generalization of the study findings needs to be done carefully. Base level cognitive functioning of these subjects before they started using cannabis was not available. Initial MoCA score was low in cannabis user group, but the cause-effect relationship could not be established with the current study design. Though we have not used specific cognitive remedial techniques, effect of other activities like reading newspapers, participating in group counselling sessions, physical activities, yoga, and meditation on improvement in cognitive functions cannot be ruled out. Long-term cohort studies with good number of participants, a sample consisting of subjects with exclusive cannabis use and a rigid study design will yield much clear results in future.

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