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Cannabis Consumers at Higher Risk of Developing Poor Mental Health, Study Suggests

By Alexander Beadle

Published: Oct 07, 2021   
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People with a history of cannabis use in their medical records appear to be at a higher risk for developing problems with anxiety, depression, and even more severe mental illnesses such as schizophrenia, according to a new study from researchers at the University of Birmingham.

Published in the journal Psychological Medicine this month, this new study examined medical records from nearly 800 general practices (GPs) around the UK between the years 1995 and 2018. The researchers found that those with a history of cannabis use were around three times more likely to develop common mental health problems, such as depression and anxiety. The cannabis-using patient group was also nearly seven times more likely to develop severe mental illnesses, such as psychosis or schizophrenia.

The researchers say that these results contradict the increasingly commonly held belief that cannabis is “one of the safer” drugs, and that public health messaging should continue to advise the public to exercise caution. Improvements to drug use prevention programs and programs designs to offer support to drug users would also help to prevent these negative effects on mental health, they say.

Cannabis users nearly seven times more likely to develop severe mental illness

The study used anonymized data drawn from the IQVIA Medical Research Database and included over 10 million eligible patient records from a 23-year period before 2018.

For this study, patient records that included a clinical code (known as a Read Code) for cannabis use were matched against two other patient records that did not report any cannabis-related Read Codes for comparison. The researchers identified 28,218 patients who had a recorded exposure to cannabis whose data was then matched to 56,208 unexposed patients with similar demographic information.

After adjusting for confounders, the researchers found that those with a history of cannabis exposure were around 2.6 times more likely to experience anxiety, 3.3 times more likely to experience depression, and 6.6 times more likely to have a more severe mental illness.

The researchers also noted that at the study entry point, nearly half of those with a history of cannabis exposure already had at least one mental health diagnosis, compared to just 20 percent of the unexposed group. However, even after excluding of those with pre-existing mental health problems from the analysis this trend continued; the exposed group were around 2.5 times more likely to develop anxiety problems, 2.3 times more likely to develop depression, and 6.4 times more likely to develop a serious mental illness.

“Cannabis is often considered to be one of the ‘safer’ drugs and has also shown promise in medical therapies, leading to calls for it be legalized globally,” senior author Dr Clara Humpston, a research fellow within the School of Psychology at the University of Birmingham, said in a statement. “Although we are unable to establish a direct causal relationship, our findings suggest we should continue to exercise caution since the notion of cannabis being a safe drug may well be mistaken.”

The study also found that those with a history of cannabis exposure tended to have higher rates of exposure to other illicit drugs, including heroin, cocaine, and amphetamines. Taken together, the researchers believe that these study results demonstrate a real need for cannabis to remain a central part of public health messaging and drug use prevention campaigns.

Dr Joht Singh Chandan, a clinical lecturer in public health at the University of Birmingham, said that “the research reaffirms the need to ensure a public health approach to recreational drug use continues to be adopted across the UK. We must continue to progress measures to improve the prevention and detection of drug use as well as implement the appropriate supportive measures in an equitable manner to prevent the secondary negative health consequences.”

The complex relationship between cannabis and mental health

This is not the first time that cannabis use has been linked to poorer mental health outcomes. In a study published last year in JAMA Psychiatry, researchers found that the frequent consumption of high-potency cannabis in young people is associated with poor mental health and addiction behaviors.

For this study, the researchers interviewed more than a thousand young people who had used cannabis within the past 12 months. The majority reported using low-potency cannabis products, such as low-THC varieties of herbal cannabis, but 12.8 percent of the study group reported using high-potency products such as hashish.

These high-strength users were significantly more likely to report having generalized anxiety disorder. But notably, no correlation was observed with reports of depression.

“When we do research into cannabis and mental health, people suggest that what we’re seeing is a kind of artefact of self-medication – that people use the drug to attenuate their own mental health symptoms,” senior study author Dr Lindsey Hines, a research fellow at Bristol Medical School, told Analytical Cannabis at the time.

“And I think the fact that we see an association with anxiety but not depression is kind of an argument against that in this case. Because, anecdotally, people use cannabis to help deal with their anxiety and depression […] So the fact that we’re only seeing this for anxiety supports the case that this is less likely to be self-medication.”

While this study was not designed to prove a causal relationship between the drug and any specific mental health conditions, it does suggest that this is an area that should be investigated in more detail.

However, cannabis has also been shown to have a positive effect on anxiety and depression in patients receiving medical cannabis treatment. Early results from Project Twenty21, Europe’s largest medical cannabis registry, suggest that medical cannabis treatment can have significant benefits to patient quality of life, with similar improvements observed in the ability of patients to manage secondary anxiety and depression when using medical cannabis.

“On a measurement of 0-to-100, where 100 presents the best imaginable quality of life, the mean score for our T21 patients [when initiating treatment] was 46.8,” Dr Anne Katrin Schlag, head of research at Drug Science, told Analytical Cannabis at the time.

“We compared our findings to normative data from the UK household population. Here, the mean was 85.7 – showing just how low quality of life is for our patients [at initiation of treatment].”

“Yet our 3-month follow up data, albeit with a small sample size of 64 so far, shows that patients' quality of life improved significantly, from 46.8 to 63.3 – a 50 percent increase in self-reported quality of life, which indicates a large effect of CBMPs for improved health.”

The Project Twenty21 team hopes that the real-world data being gathered and analyzed as a part of this project will help to provide a solid evidence base supporting the safety and efficacy of medical cannabis products, which could eventually encourage improved access to medical cannabis through Britain’s National Health Service.


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