Cannabis may help treat anxiety and depression. But there’s still plenty of unknowns
US researchers find cannabis shows promise of being an effective antidepressant, but their study has several key limitations.
For anyone struggling with anxiety or depression, there is no such thing as an ideal treatment option. Talking therapies work about half the time, but they're not always available, affordable or convenient. Antidepressant drug treatments also work about half the time, but some people might be put off by potential side effects, such as sickness and loss of appetite, and the risk of what's been called a 'discontinuation syndrome' or 'withdrawal syndrome' – including flu-like symptoms and insomnia. For these reasons, researchers are always on the look-out for alternative treatment options.
According to a new study carried out at the Medical University of South Carolina published in Frontiers in Psychiatry, one promising candidate in this regard is cannabis – a drug that many states in the US have already licensed for other medicinal uses, though it remains illegal in the UK.
If you've ever taken cannabis recreationally and experienced a high or a calming haze, you'll see why the drug might have potential as an antidepressant or anti-anxiety treatment. Indeed, research carried out at the Community Forensic Mental Health Service, Brisbane, Australia into the subjective effects of cannabis show that feelings of euphoria, relaxation and pain relief are a common experience.
Added to that, there's also mouse research that's shown the possible antidepressant effects of cannabis. These kind of studies typically involve the mouse being made to do something stressful (such as an exhausting swim) to model the effects of feeling depressed. One study carried out by a team at the University of São Paulo, Brazil has shown that giving the mice cannabis after the stressful experience has a kind of therapeutic effect for them.
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It's important to bear in mind that cannabis has two main ingredients – delta-9-tetrahydrocannabinol (THC), which is responsible for the high, and cannabidiol (CBD), which has a relaxing effect and which you might have seen being sold as a food supplement added to coffee or brownies - though usually in tiny amounts that are unlikely to have much of an effect.
Part of the appeal of cannabidiol, in particular, as a potential treatment for depression and anxiety is that it appears less likely to induce the kind of withdrawal effect seen with existing anti-depressant drugs.
For the new study, the US team led by Erin Martin recruited hundreds of participants who showed symptoms of depression and/or anxiety and were either already taking cannabis for medicinal reasons to help relieve their symptoms or were interested in doing so - the latter group acted as controls. The researchers then completed further surveys over the next few years to see whether any of the controls had begun taking cannabis, and whether any of the participants' anxiety and depression treatments had changed.
The most promising findings were for depression – the participants who were using cannabis at the start of the study reported lower depression symptoms than those who weren't. What's more, they also reported better sleep, less pain and all round better quality of life.
Perhaps even more compelling is the finding that those participants who began using cannabis later in the study then also showed reductions in their depression symptoms compared with the participants who did not start using cannabis - in fact, this latter group showed no change in their symptoms from their baseline scores at the start of the study. Martin and her team found these results encouraging: “These combined cross-sectional and longitudinal findings show a consistent anti-depressant effect of medicinal cannabis,” they wrote.
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The results for anxiety were less striking but still positive. Although there were no differences in baseline anxiety symptoms between participants taking cannabis at the study start and those who weren't, the participants who began to use cannabis later in the study went on to report reduced anxiety symptoms compared with the participants who never used cannabis.
The study has prompted news headlines around the world suggesting that cannabis could be a useful treatment for depression and improving quality of life, so how excited should we be?
It's important to note that this was not a properly controlled, randomised trial – in fact it wasn't even close. The participants were recruited on the basis of being interested in exploring the medicinal benefits of cannabis which ramps up the likelihood of so called 'expectancy effects' or placebo effects – which is exactly the kind of thing that proper trials are designed to control for. The researchers acknowledge this in their paper – "Placebo-controlled clinical trials are necessary to further explore the potential efficacy of CBD in the treatment of anxiety and depression," they write.
The researchers also relied on the participants' testimony for just about everything – whether or not they were using cannabis, what kind it was, and any effects it might have had. In fact, over a quarter of the participants using cannabis in this study said they didn't actually know what kind of cannabis it was. Most of those who were aware reported that their cannabis was CBD-dominant.
However, we don't know what doses the participants in this study were using, nor the role played by any other ingredients in their particular cannabis. Similarly, if you buy cannabis on the street, the chances are you won't know its strength nor what balance of THC and CBD it has. What's more, a typical cannabis leaf will contain dozens of other ingredients that may have their own physiological effects.
In short, even if the positive results in this study are real, we're still a very long way from knowing what kind of cannabis, and in what dose, would be optimal for treating depression and anxiety. Added to that, it's worth remembering that cannabis use has been linked with increased risk of psychosis - especially for those people with an existing vulnerability. There's also evidence from a study by the Centre for Addiction and Mental Health, Toronto, Ontario, Canada that THC-heavy forms of cannabis - the kind you're likely to take recreationally - could actually contribute toward increased risk of depression over the long term, though the jury is still out on that question.
Overall, while these new results are promising, and they offer hope of a new treatment options for treatment and anxiety in the future, we're still a long way from the day that your GP prescribes you a course of spliffs to boost your mood or calm your nerves.
WARNING: Cannabis is a Class B drug according to UK law. Anyone caught in possession of such substances will face up to five years in prison, an unlimited fine, or both. More information and support for those affected by substance abuse problems can be found at bit.ly/drug_support
Dr Christian Jarrett is a cognitive neuroscientist, science writer and author. He is the Deputy Editor of Psyche, the sister magazine to Aeon that illuminates the human condition through psychology, philosophy and the arts. Jarrett also created the British Psychological Society's Research Digest blog and was the first ever staff journalist on the Society's magazine, The Psychologist. He is author of Great Myths of The Brain and Be Who You Want: Unlocking the Science of Personality Change.