Teenagers and young adults should avoid trying cannabis, academics have said following a major new review.
Drivers should also avoid the drug, as should pregnant women and people with mental health illnesses, they said.
Researchers conducted an umbrella review of studies which have been published into cannabis use to determine which groups may benefit from cannabis and which groups should avoid it.
The new research, published in the BMJ, found that while cannabis compounds could be helpful for people with certain medical conditions, taking the drug could be detrimental for certain groups of people.
An international team of 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.
This 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, they said.
The authors said that cannabis should be avoided among young people while their brains were still developing.
The researchers 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”.
But they said that cannabidiol was beneficial for people with epilepsy to help them avoid seizures.
And 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 they 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,” the authors 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 comes as a separate study found that marijuana users had “significant levels” of metals in their blood and urine.
The study, published in the journal Environmental Health Perspectives, saw the blood and urine of 7,254 people in the US analysed.
Academics from Columbia University Mailman School of Public Health, who led the study, said that marijuana may 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, 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, but the nhs.uk website warns “there’s no guarantee these are of good quality or provide any health benefits”.