Abstract

Cannabis use is observationally associated with an increased risk of schizophrenia, but whether the relationship is causal is not known. Using a genetic approach, we took 10 independent genetic variants previously identified to associate with cannabis use in 32 330 individuals to determine the nature of the association between cannabis use and risk of schizophrenia. Genetic variants were employed as instruments to recapitulate a randomized controlled trial involving two groups (cannabis users vs nonusers) to estimate the causal effect of cannabis use on risk of schizophrenia in 34 241 cases and 45 604 controls from predominantly European descent. Genetically-derived estimates were compared with a meta-analysis of observational studies reporting ever use of cannabis and risk of schizophrenia or related disorders. Based on the genetic approach, use of cannabis was associated with increased risk of schizophrenia (odds ratio (OR) of schizophrenia for users vs nonusers of cannabis: 1.37; 95% confidence interval (CI), 1.09–1.67; P-value=0.007). The corresponding estimate from observational analysis was 1.43 (95% CI, 1.19–1.67; P-value for heterogeneity =0.76). The genetic markers did not show evidence of pleiotropic effects and accounting for tobacco exposure did not alter the association (OR of schizophrenia for users vs nonusers of cannabis, adjusted for ever vs never smoker: 1.41; 95% CI, 1.09–1.83). This adds to the substantial evidence base that has previously identified cannabis use to associate with increased risk of schizophrenia, by suggesting that the relationship is causal. Such robust evidence may inform public health messages about cannabis use, especially regarding its potential mental health consequences.

It has been suggested that cannabis use increases risk of driving collisions through affecting braking and choice reaction time; causing impairment of lane position, disrupting head-way and dynamic tracking; distorting time perception; reducing divided and sustained attention; and increasing caution. 

A recent Canadian study, published just two days before cannabis was legalized across the country, investigated the impact of cannabis use on driving performance in young recreational users.

The study investigated 45 recreational marijuana users between the ages of 18 and 24. Participant’s driving reflexes were tested in a driving simulator with no cannabis in their system, and then one, three and five hours after inhalation of a standard 100-mg dose of cannabis through a vaporizer (a typical joint is 300-500 mg of dried cannabis). 

Participants were also asked to rate perceived driving ability and safety at each stage of the experiment.

Results found that although cannabis use made no significant difference when it came to simple driving actions such as braking, steering and maintaining a steady speed, complex driving-related performance, such as reacting to avoid sudden obstacles, was affected at all time points after cannabis use. 

Findings from the self-reported driving ability and driving safety showed that participants in this trial did not perceive themselves to be as safe to drive at 1, 3 and even 5 hours after cannabis use.

Overall the research highlights the importance of adhering to the guidelines on driving after using cannabis and waiting at least 6 hours before getting behind a wheel.

The controversial subject of cannabis legalisation has led to the increased urgency in understanding the full health and social implications of the drug use. Cannabis use is particularly prevalent amongst adolescence - a period of time where young people are going through significant neural development. Cannabis use is thought to affect normal neuromaturation, increasing the risk of cognitive impairment. 

A recent study, published in the Journal of Clinical Psychiatry, has investigated whether young people can recover from drug-induced cognitive impairment by stopping cannabis use for an extended period of time.

The study analysed 88 young people, aged between 16 and 25, who described using cannabis on a weekly basis. The participants were grouped into those who were asked to abstain using cannabis for one month and those who were allowed to continue use. The participants were also asked to complete multiple cognitive tests over the course of the month.

Results found that memory (specifically declarative memory - the ability to learn and recall new information) improved only among the group who stopped using marijuana for a month. The researchers concluded that cannabis use disrupts areas of the brain that are particularly involved in the memory-learning network and that abstaining from cannabis allows for more efficient processing of information. 

This research provides evidence that adolescents may experience improvements in their capacity to learn and recall new information when they stop using cannabis.