In Depth

The truth about the health effects of cannabis

New research links teen marijuana use to depression and suicide in adulthood

Teenagers who use marijuana are increasing their risk of depression and suicidal thoughts in adulthood,  new research suggests.

A team of British and Canadian scientists analysed the results of 11 studies dating back to the 1990s involving a total of more than 23,300 people worldwide. After taking into account factors including mental health issues at the outset and socioeconomic status, the researchers found that the odds of attempting suicide in early adulthood were “almost 3.5 times worse among those who used cannabis before the age of 18 than those who did not”, reports The Guardian.

People who had used the drug as teenagers were also 37% more likely to go on to develop depression, according to results published in JAMA Psychiatry.

The researchers estimate that about 60,000 cases of depression in adults aged under 35 in the UK, and more than 400,000 in the US, could be avoided if adolescents did not smoke cannabis.

Study co-author Dr Andrea Cipriani, from the University of Oxford’s department of psychiatry, said: “The number of people who are exposed to cannabis, especially in this vulnerable age, is very high and I think this should be a priority for public health and the medical sector as well.”

Nevertheless, debate continues over whether legalising cannabis would be beneficial.

Advocates say allowing cannabis to be sold for recreational use would create new jobs and businesses, and save resources in the police and criminal justice system.

On the other side of the argument, anti-legalisation campaigners warn that the move risks normalising the use of drugs among children and would increase levels of addiction and associated health problems.

Recreational use of cannabis is already permitted in ten US states and in Canada, and many European countries are moving towards decriminalisation. 

Critics warn that this legalisation sends the message that the psychoactive drug is safe to use - but what are the health risks, and benefits, associated with with marijuana use?

What do supporters say?

Cannabis has long been promoted as an effective treatment for a host of medical conditions, including epilepsy and chronic pain.

The California Medical Association is among a number of doctors’ groups that have called for the full legalisation of marijuana, arguing that the drug “should be strictly regulated like medicine” to ensure safe and appropriate use by patients with legitimate health problems.

Supporters also argue that legal recreational drugs such as alcohol and tobacco pose a far greater risk.

“In several respects, even sugar poses more of a threat to our nation’s health than pot,” says clinical psychologist David Nathan in an article for the CNN website.

That view is echoed by reporter Christopher Ingraham, who covers drug policy for The Washington Post. It takes “extraordinary chutzpah to rail against the dangers of marijuana use by day and then go home to unwind with a glass of far more lethal stuff in the evening”, Ingraham writes.

Legalising cannabis could have considerable financial benefits for national coffers, too. The move would raise £1bn a year in tax for the UK alone, according to the Institute of Economic Affairs (IEA), which estimates that the black market in the drug is worth about £2.6bn a year. The think tank has also said that legalisation would lead to savings for police and other public services.

“Done properly, the legalisation of cannabis is a win-win-win - criminals lose a lucrative industry, consumers get a better, safer and cheaper product, and the burden on the general taxpayer is reduced,” the IEA’s head of lifestyle economics, Chris Snowdon, told the BBC.

What do critics say?

In the Los Angeles Times, political columnist George Skelton asks: “Why the hurry to legalise consumption of another poison? We’ve already got alcohol, which can ruin lives. Tobacco causes cancer. Cannabis? It can mess up the mind. Plenty of research shows that.” 

Opponents also claim that the potency of the drug has increased dramatically in recent years, especially when consumed in food and drinks.

Modern marijuana “isn’t like the crappy pot you were smoking in college”, says Andrew Acosta, spokesperson for the “No on 64” campaign, which fought against legalisation in California.

“This is high-grade stuff they’re putting in candy bars and you’re climbing the wall.” 

Despite being listed as a Schedule 1 drug in the US - a classification that defines a drug as having “no currently accepted medical use and a high potential for abuse” - marijuana and its many derivatives have been found to be beneficial in treating or alleviating some health conditions.

The US National Academy of Sciences, Engineering and Medicine conducted a comprehensive review of 10,000 research papers in 2017, for one of the most wide-ranging studies yet on the health effects of recreational and therapeutic cannabis use.

The review found “conclusive or substantial evidence” that cannabis or its other compounds, known as cannabinoids, can be an effective treatment for chronic pain, muscle spasms associated with multiple sclerosis, and chemotherapy-induced nausea and vomiting.

A separate clinical trial published in The New England Journal of Medicine in 2017 found cannabidiol, a marijuana derivative, to be beneficial in the treatment of a complex childhood epilepsy disorder.

Gregory Gerdeman, a neuroscientist who specialises in the physiological actions of cannabis, says it is “already widely understood that marijuana is valuable and safe as a palliative medicine, which undermines the tenets of the Schedule 1 status”, reports Time magazine.

Additionally, there are anecdotal patient reports, increasing numbers of clinical case studies and a wide range of pre-clinical research that indicate cannabinoids could help shrink some of the most serious types of tumours, Gerdeman said. 

However, scientists say more research is needed to determine whether the drug can serve as an effective cancer therapy.

What about the risks?

Cannabis is roughly 114 times less deadly than alcohol, according to research published in 2015. Nevertheless, “safer than alcohol” doesn’t mean “safe, full stop”, warns The Washington Post’s Ingraham.

One of the biggest concerns is marijuana’s effect on mental health. The NHS website warns that regular cannabis use increases the user’s risk of developing a psychotic illness, such as schizophrenia. The risk increases further if that person started using cannabis at a young age, smokes stronger types of the drug, such as skunk, or smokes regularly or for a long time, the health service says.

The NHS also suggests that heavy cannabis users are more likely to develop bronchitis, although other experts have drawn varying conclusions. Some studies point to the positive effects of  tetrahydrocannabinol (THC), the principal psychoactive ingredient of cannabis, on opening the airways, while others highlight the negative outcomes of marijuana smoke inhalation, according to the US Lung Institute.

Ultimately, says scientists, far more research into the drug is needed.

“It’s actually quite amazing how little we really know about something that has been used for thousands of years,” says Sachin Patel, of Tennessee’s Vanderbilt University, who studies cannabis.

“We desperately need well-controlled unbiased large-scale research studies into the efficacy of cannabis for treating disease states, which we have very little of right now,” Patel told Time. 

So what is the consensus?

The evidence suggests cannabis is a relatively safe drug that provides a host of medical benefits, but that it is not harmless and that its potential risks to physical and mental health should not be ignored. More research is needed to determine whether or not the long-term risks outweigh the rewards.

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