Cannabis is legal for adults in countries, and legal status changes how people see it. A product sold in a licensed shop seems safer than one bought on the street, and sometimes that is true. Legalization changes access, price, advertising and social acceptance. It does not change what the drug does to the brain, and that difference matters for anyone deciding whether to use it.
A recent cohort study in JAMA Psychiatry asked a similar question about online sports betting: when it became legal, did people use more behavioral health care? The same question applies to cannabis. When something becomes easier to obtain, some people develop problems, and clinics, family doctors and families usually notice only after a delay.
What Cannabis Does to Mood, Memory and Thinking
THC is the compound mainly responsible for the high. It acts on receptors in the brain that help regulate mood, memory, appetite and perception. In the short term it can bring relaxation and talkativeness. It can also bring anxiety, a racing heart, slower reaction time and poor attention. Memory for recent events is impaired while the person is intoxicated, and driving is less safe.
Starting young seems to carry more risk. In a review of the health effects of marijuana, Volkow and colleagues (2014) described links between regular use that begins in adolescence and addiction, poorer school performance and problems with attention and memory. Some of these links may be partly explained by other factors, such as difficulties that were already present before the person started using.
Mood is another concern. Gobbi and colleagues (2019) pooled results from 11 studies that followed young people over time. Those who used cannabis before age 18 had higher odds of depression in young adulthood, with a pooled odds ratio of about 1.4. They also had higher rates of suicidal thoughts and attempts. The increase for anxiety was not clear. These studies are observational, so they cannot prove cause.
The strongest concern is psychosis, a state in which a person loses touch with shared reality through hallucinations or fixed false beliefs. Di Forti and colleagues (2019) compared people with a first psychotic disorder to healthy people at 11 sites in Europe and Brazil. Daily cannabis use was linked to about three times higher odds of psychosis, and daily use of high-potency cannabis to about five times higher odds.
That study was a case-control design, which cannot fully separate cause from effect. A person in the early, unrecognized phase of psychosis might also use more cannabis. Still, the risk rose with frequency and potency, and it differed between cities according to the strength of the cannabis sold there. Most people who use cannabis never develop psychosis, but the risk is real and concentrated in heavy use.
When Use Becomes a Problem
Cannabis can lead to dependence. Leung and colleagues (2020) reviewed the research and estimated that roughly 13 percent of people who use cannabis meet criteria for cannabis use disorder. The risk is higher with daily use and with early starts. Hasin and colleagues (2017) compared US national surveys from 1991 to 1992 and 2012 to 2013 and found that both use and the disorder became more common among adults.
Surveys of this kind show that a change happened, but they cannot show which factor caused it. Laws, attitudes and product strength all changed during those years. For one person, the useful question is simpler: is cannabis taking more time, money and attention than it should, and is life getting smaller around it?
Daily use, high-THC products and starting in adolescence are the patterns linked to the greatest risk in the research described here. Legal status tells you who may buy cannabis, not whether it is safe for you.
The signs of a problem are fairly consistent. A person uses more than planned, tries to cut down and cannot, or spends a lot of time getting and recovering from the drug. Cravings appear. Work, school or relationships suffer, yet use continues. When use stops, irritability, poor sleep, low appetite and restlessness can start within days and usually ease within two weeks.
Many patients tell me they use cannabis to sleep or to calm anxiety. It may help in the moment, but the evidence that it treats either condition over the long term is limited. After regular use, sleep often gets worse for a while when a person stops, which makes people think they need it. Treatments with better evidence exist for both problems.
Practical Steps for Lowering Risk
A good first step is to keep a record for two weeks. Write down the days you used, how much, the THC strength on the label, and your reason, such as boredom, stress or trouble sleeping. Add your mood that evening and the next morning. Most people find patterns they had not noticed, and the record gives a doctor useful facts.
Some people should avoid cannabis entirely. These include anyone under 25, because the brain is still developing, and anyone who has had psychosis or has a parent or sibling with a psychotic disorder. Pregnant and breastfeeding women should also avoid it. Everyone else can lower risk by using less often, choosing lower-THC products and avoiding daily use.
Do not drive for several hours after using, and wait longer after edibles, because their effects start later and last longer. Do not mix cannabis with alcohol or sedating medicines. Be cautious with edibles, since a person who feels nothing after an hour may take more and then become far more intoxicated than planned.
If you want to cut down and cannot, tell your doctor. The treatments with the best evidence are talking therapies, including cognitive behavioral therapy and motivational interviewing, and programs that reward days without use. No medication is approved specifically for cannabis use disorder. Tell the doctor about withdrawal symptoms, because poor sleep and irritability are the usual reasons people start again.
Seek help quickly if cannabis use is followed by suspiciousness, hearing voices, or thoughts of self-harm. In the United States, you can call or text 988 at any time. Family members can raise the subject calmly and with specific examples, at a time when the person is sober. Arguments during or just after use rarely lead anywhere.
Studies like the one on sports betting show that legalization can be followed by changes in the use of care, and that these changes can be measured. For cannabis, the data are still accumulating. In the meantime, the evidence supports a simple position: most of the harm is found in frequent use, strong products and early starts, and each of these can be changed.
References
- Di Forti, M., Quattrone, D., Freeman, T. P., Tripoli, G., Gayer-Anderson, C., Quigley, H., et al. (2019). The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): A multicentre case-control study. The Lancet Psychiatry, 6(5), 427-436.
- Gobbi, G., Atkin, T., Zytynski, T., Wang, S., Askari, S., Boruff, J., et al. (2019). Association of cannabis use in adolescence and risk of depression, anxiety, and suicidality in young adulthood: A systematic review and meta-analysis. JAMA Psychiatry, 76(4), 426-434.
- Hasin, D. S., Sarvet, A. L., Cerdá, M., Keyes, K. M., Stohl, M., Galea, S., & Wall, M. M. (2017). US adult illicit cannabis use, cannabis use disorder, and medical marijuana laws: 1991-1992 to 2012-2013. JAMA Psychiatry, 74(6), 579-588.
- Leung, J., Chan, G. C. K., Hides, L., & Hall, W. D. (2020). What is the prevalence and risk of cannabis use disorders among people who use cannabis? A systematic review and meta-analysis. Addictive Behaviors, 109, 106479.
- Volkow, N. D., Baler, R. D., Compton, W. M., & Weiss, S. R. B. (2014). Adverse health effects of marijuana use. New England Journal of Medicine, 370(23), 2219-2227.
