What Determines Whether Cannabis Improves or Worsens Your Mental Health

One of the applications of medical cannabis is for the improvement of mental health. In the UK (where I’m from), you can be prescribed medical cannabis to counteract the symptoms of a range of mental health problems, including depression, anxiety, and PTSD. Private cannabis clinics are seeing a rise in the number of patients who are seeking cannabis to treat their ailments.

Most medical cannabis patients (49%) use the drug for pain conditions, but in a close second are those (42%) using it to improve their mental health.

There is no shortage of accounts of people whose mental health has improved – sometimes significantly – after using medical cannabis. Where other traditional medications failed (or worsened things), cannabis appears to act as a more effective alternative, with fewer side effects. But we need to contrast these accounts with the wealth of evidence and reports of cannabis worsening people’s mental health.

In this post, I would first like to present the research on how cannabis can either improve or worsen mental health, and then explore the factors that contribute to these outcomes.

In terms of the scientific literature, the research is not always as glowing as the accounts of medical cannabis patients. For example, a 2020 systematic review and meta-analysis published in The Lancet Psychiatry concluded, based on 83 studies:

There is scarce evidence to suggest that cannabinoids improve depressive disorders and symptoms, anxiety disorders, attention-deficit hyperactivity disorder, Tourette syndrome, post-traumatic stress disorder, or psychosis. There is very low quality evidence that pharmaceutical THC (with or without CBD) leads to a small improvement in symptoms of anxiety among individuals with other medical conditions.

THC- and CBD-based medicines, given as adjunct to pharmaco- and psychotherapy, were associated with improvements of several symptoms of mental disorders, but not with remission. Side effects occurred, but severe adverse effects were mentioned in single cases only.

This is just a selection of the evidence so far, but I focused on systematic reviews and meta-analyses because they pool many studies together. And what they show is that the evidence on the mental health benefits of medical cannabis is often mixed, limited, or weak; the use of cannabis may be more helpful for some mental health problems than others. While some researchers find little evidence that cannabis helps depression, other studies (see here and here) show use is linked to strong antidepressant effects; although it isn’t risk-free, as there is always a chance that such use sustains or induces a cannabis use disorder (the official term for cannabis addiction).  

The evidence so far suggests that the type of cannabis used (low vs. high strength) and how it is used (e.g. alone or in conjunction with psychotherapy) influences mental health outcomes. (More on this in a later section.)

A 2023 review of meta-analyses of randomised controlled trials and observational studies, published in the BMJ, concluded that “Convincing or converging evidence supports avoidance of cannabis during adolescence and early adulthood, in people prone to or with mental health disorders”. Of particular concern was the link between heavy or frequent cannabis use in adolescents and psychotic symptoms. Previous research noted this risk, including the risk factors:

Age of onset of cannabis use, frequent cannabis use, exposure to childhood trauma, concurrent use of other substances and genetic factors.

Many people who have used cannabis – especially heavily/frequently – may challenge the idea that there is no link between cannabis and depression or anxiety. It worsens both for many users, leading them either to continue using (a sign of addiction), cut down, or stop completely. In addition, a 2024 systematic review found that “cannabis use is associated with increased depressive and manic symptoms in the general population in addition to an elevated likelihood of developing MDD [major depressive disorder] and BD [bipolar disorder].”

The authors behind a 2024 study state:

The association between cannabis and anxiety is best explained by anxiety predisposing individuals toward CU [cannabis use] as a method of self-medication. A causal relationship in which CU causes AD [anxiety disorder] incidence is less likely despite multiple longitudinal studies suggesting so.

None of this is to discount the mental health benefits that medical cannabis patients report. The above studies on mental health risks, we should remember, involve recreational use. This kind of use is often dissimilar from the way in which medical cannabis patients use the drug. At the same time, people outside of the medical system find that they can use cannabis in a way that reduces their psychological distress. For this reason, we should now turn to the factors that influence whether cannabis can harm or help people.

We’ve already identified some of the risk factors linked to the detrimental effects of cannabis: use in adolescence, frequent use, heavy use, and high amounts of THC. Regarding this last factor, it’s important to stress that pure THC – that is, without any CBD – has a tendency to induce psychotic-like states. But the risk of experiencing psychotic symptoms (e.g. paranoia), or other signs of worsened mental health, is not eliminated by having minimal amounts of CBD present.

High-strength cannabis (with high amounts of THC and low amounts of CBD) has a greater tendency to worsen mental health than low-strength cannabis.

There is evidence suggesting that CBD has antipsychotic properties. It is seen to protect users against the negative effects of THC. This has been confirmed by neuroimaging research. While THC is responsible for the desirable psychoactive effects of cannabis, it is also what causes its less desirable effects. The relationship between THC and CBD can help explain why high-potency cannabis is riskier to use than low-potency cannabis. Thus, there is an argument to be made that legalising cannabis for commercial sale should occur with more careful regulation, including setting limits on the concentration of THC in commercially available products and/or a certain concentration of CBD that should be present in products. However, this involves a debate between a libertarian position that might imagine all kinds of potencies available, with adults given the freedom of choice to decide what to use, and a harm reduction (but still liberal) position that tries to balance freedom with safety.

Many people would like to have better experiences with cannabis; but currently, a system of prohibition means that a lot of the cannabis available is of the high-strength variety. This makes sense from the sellers’ point of view: a higher-strength product is more profitable. If it tends to make people more addicted, then they keep coming back for more. But, as we’ve seen, it is also linked to a greater incidence of mental health problems. People who may have previously had positive experiences with cannabis (perhaps at a time when it wasn’t so strong), or who sometimes enjoy it, might have to forgo it because they are limited by the kind of cannabis available in the black market. And those who continue to use it anyway may suffer (sometimes serious) negative consequences.

Anecdotally, many people who have used psychedelics also find that cannabis feels different after their trips. It can feel more psychedelic, but this change is often unwelcome.

In a legal, regulated system in which ratios of THC and CBD were more controlled, with psychological health in mind, many dangers of cannabis use could be mitigated, and benefits associated with use made more likely. (Legalisation does not, in itself, eliminate the risks of cannabis; in US states that have legalised cannabis, there is still the problem of high-potency products dominating the market.) A 2018 study showed that low THC/high CBD cannabis was best for reducing self-reported symptoms of depression, while, interestingly, high THC/high CBD cannabis was tied to reductions in stress. The antidepressant effect occurred after just one puff. (Both of these effects were short-term, however; the authors underscore that continued use may exacerbate depression over time.)

Researchers who found that medical cannabis use improves depression, sleep, and overall quality of life emphasise that these benefits tend to occur when products high in CBD are used. An earlier study, published in 2007, revealed that THC can act as a potent antidepressant in low doses, but in higher doses, it can worsen depression and other conditions like psychosis. So if you want to use cannabis to improve your mental health, it’s best to stick with low THC/high CBD products. Of course, it’s easier to access such products if they’re medically prescribed or commercially available (since you have greater choice over what to buy, and information on THC/CBD ratios). In a black market, you’re just putting your trust in whatever the dealer says about what they’re selling, and it may take some trial and error (and potentially negative cannabis experiences) before you find a product that is a suitable strength.

For those without a medical cannabis prescription or access to legal recreational cannabis, one option is to mix CBD weed (which is legal in many countries since it contains 0.2% THC or less) with a small amount of regular weed (which is likely to be of the high-potency variety).

Moreover, if you’ve had a personal history of being addicted to weed, you should be conscious of your patterns of use, such as whether the use of low-potency weed shifts to higher-potency weed, and if occasional or infrequent use turns into heavier use. Other signs of unhealthy use include strong cravings and increasingly using cannabis on one’s own, instead of socially, as well as becoming more reclusive in general. If you struggle to control your use, it may be time to step back and explore why this is the case. It’s possible you could return to a healthy pattern of cannabis use if you’re able to address the root cause of the addiction. But for some people, the best decision for their mental health is to avoid cannabis entirely.

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