Teenagers, young adults, pregnant women, drivers and mentally ill people should avoid cannabis, according to the largest ever health review of its kind.
However, cannabidiol can help reduce seizures in epilepsy patients, and cannabis-based medicines may help with multiple sclerosis, chronic pain, inflammatory bowel disease and in palliative care.
The findings were based on an umbrella review conducted by an international expert team of gold standard studies on cannabis and health carried out over the last two decades. Umbrella reviews look at previous meta-analyses and provide a summary of evidence on a particular topic.
The in-depth evidence review of cannabis and health was published in the BMJ.
It found that while cannabis compounds could be helpful for people with certain medical conditions, taking the drug could be detrimental for other groups of people.
The experts, including researchers from the UK, analysed data from 101 meta-analyses on cannabis use. The studies were published from 2002 to 2022 and looked at the effects of different combinations of cannabis, cannabinoids and cannabis-based medicines on health.
The review of reviews concluded that cannabis use was linked to poor mental health and cognition. It increased the risk of car crashes among drivers and led to poor outcomes for babies when pregnant women used the drug.
The authors said that cannabis should be avoided among young people while their brains were still developing. They argued that most mental illnesses were first identified during teenage years and young adulthood. And this was also a period when “cognition is paramount for optimising academic performance and learning”.
However, they said cannabidiol was beneficial for people with epilepsy to help them avoid seizures.
Cannabis-based medicines could also help reduce chronic pain and could help reduce spasms among people with multiple sclerosis. It could also help reduce nausea and vomiting among patients with a range of conditions and help improve the sleep of cancer patients.
Cannabis-based medicines were also found to improve quality of life among patients with inflammatory bowel disease and were found to be effective in palliative care. But the authors stressed that the use of cannabis-based medicines were “not without adverse events”.
“Convincing or converging evidence recommends avoiding cannabis during adolescence and early adulthood in people prone to have or have mental health disorder, who are pregnant, and while driving,” they wrote.
“Cannabidiol is effective for epilepsy, notably in children, while other cannabinoids can be effective in use for multiple sclerosis, chronic pain, inflammatory bowel disease, and palliative care.”
It came as a separate study found that marijuana users had “significant levels” of metals in their blood and urine. For the study, published in the journal Environmental Health Perspectives, the blood and urine of 7,254 people in the US was analysed.
Academics from Columbia University Mailman School of Public Health, who led the study, said marijuana could be an under-recognised source of lead and cadmium exposure among users.
Cannabis in general is not legal in the UK and it is known as a class B drug. But medicinal cannabis – or cannabis-based medicines – can be used.
Specialist doctors can prescribe medicinal cannabis for conditions such as severe epilepsy and for cancer patients suffering side-effects from certain drugs and patients with multiple sclerosis.
People can also purchase products such as CBD oil or hemp oil. However, the NHS website says “there’s no guarantee these are of good quality or provide any health benefits”.