When you have over 2 million people trying cannabis in the last year, the odds are that some of them already have or will go on to have mental health problems such as psychosis.

It would be handy to know who is the most likely to develop such a problem. Science should be able to provide some clues as researchers first began looking into the link between cannabis and psychosis in the Seventies, so we’ve had nearly 50 years to figure this one out. But science rarely churns out binary answers to questions and the cannabis psychosis conundrum is no different.

What has become clearer is that the more cannabis you smoke, the greater your risk of becoming psychotic – this is often referred to as the “dose-response” relationship. This means that a small group of people face the highest risk, as a mere 9 per cent of users get through 73 per cent of all the cannabis consumed. Also, for people who already have schizophrenia, cannabis can make the hallucinations and thought disorder they experience worse; they also take longer to recover from these when they do get treatment.

To get a sense of the size of the risk, we would need roughly 23,000 people to abstain from ever using cannabis to prevent one of them from becoming psychotic. That’s based on the research carried out to date. But the research isn’t perfect: one particular flaw stands out – most of it was conducted when people were using lower potency cannabis like resin.

There have been no studies large enough or sophisticated enough since higher potency versions of cannabis like skunk came along. Higher potency cannabis contains less cannabidiol (CBD), which is believed to offer some protection from developing problems such as psychosis, but higher levels of tetrahydrocannabinol (THC), which can trigger psychosis. So it’s possible that we might need fewer people to abstain from cannabis to stop one of them developing a mental health problem if higher potency strains increase the risk. But we just don’t know.

In pictures: 4/20 Marijuana world rallies Show all 26 1 /26 In pictures: 4/20 Marijuana world rallies In pictures: 4/20 Marijuana world rallies A man wears a marijuana leaf mask during the annual 4/20 cannabis culture celebration at Sunset Beach in Vancouver, British Columbia AP In pictures: 4/20 Marijuana world rallies A lady smokes marijuana on Parliament Hill on 4/20 in Ottawa, Ontario Getty Images In pictures: 4/20 Marijuana world rallies James Reed smokes a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies People sign a 4/20 sign on Parliament Hill on in Ottawa, Ontario Getty Images In pictures: 4/20 Marijuana world rallies A woman smokes marijuana on Parliament Hill on 4/20 in Ottawa, Ontario. Polling released showed strong support in Canada for a government drive to legalise recreational use of marijuana, but many would like the proposed minimum age for consumption to be raised. Sixty-three percent of respondents told the Angus Reid Institute they support legalisation Getty Images In pictures: 4/20 Marijuana world rallies A man smokes marijuana during the annual 4/20 marijuana rally on Parliament Hill in Ottawa, Ontario, Canada Reuters In pictures: 4/20 Marijuana world rallies Demonstrators smoke marijuana during the '4/20 Santiago' rally in favour of legalisation in front of the La Moneda presidential palace in Santiago, as part of the Global Marijuana March which is being held in hundreds of cities worldwide Getty Images In pictures: 4/20 Marijuana world rallies People play with a mock marijuana joint during a 4/20 party to demand legalisation and to celebrate marijuana culture outside the Senate building in Mexico City, Mexico Reuters In pictures: 4/20 Marijuana world rallies Adam Eidinger, co-founder of DCMJ, hands out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC Getty Images In pictures: 4/20 Marijuana world rallies In pictures: 4/20 Marijuana world rallies Police arrest Rachel Ramone Donlan after she handed out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC AFP/Getty Images In pictures: 4/20 Marijuana world rallies Police arrest Rachel Ramone Donlan after she handed out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC Getty Images In pictures: 4/20 Marijuana world rallies Thousands of people gather to smoke marijuana during the '420 Santiago'rally in front of the La Moneda presidential palace in Santiago Getty Images In pictures: 4/20 Marijuana world rallies People attend the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Kevin Barron and Lasean Moore of Raleigh, North Carolina, share a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Nic Ruhl takes a pull on a giant hand rolled joint at precisely 4:20pm MDT during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Various cannabis paraphernalia on display at a vendor's stall during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Mo Banez, of Austin, Texas, lights a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies A man displays a large container of cannabis during the Denver 420 Rally at Civic Center Park in Denver, Colorado. The rally, held annually, is a celebration of both the legalisation of cannabis and cannabis culture. Colorado is one of twenty-six U.S. states along with the District of Columbia that has legalised the use of cannabis either recreationally or medically AFP/Getty Images In pictures: 4/20 Marijuana world rallies Sitting in small groups on mats shaded by trees in the Rose Garden just across from the Knesset, participants lit up as the clock struck 4:20 for the local version of the traditional worldwide April 20 pro-marijuana events, known as '420' rallies Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem during a rally at the Rose garden Getty Images In pictures: 4/20 Marijuana world rallies Sitting in small groups on mats shaded by trees in the Rose Garden just across from the Knesset, participants lit up as the clock struck 4:20 for the local version of the traditional worldwide April 20 pro-marijuana events, known as '420' rallies Getty Images In pictures: 4/20 Marijuana world rallies An Israeli girl poses with a mock marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current laws Getty Images In pictures: 4/20 Marijuana world rallies aelis pass around a marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current laws Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current law Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem on April 20, 2017 during a rally opposite the Knesset to celebrate 420 and express defiance of current laws AFP/Getty Images

Trying to figure out what difference, if any, the change in cannabis potency has made to rates of psychosis and schizophrenia some people suggest looking at the rate of schizophrenia and psychosis in the population over time. Initially this makes sense, but this data is problematic and influenced by its own set of unique problems which makes any interpretation difficult.

So, although the science is messy, there are things we can do to help reduce the potential for harm. A survey carried out for The Independent last year showed public opinion warming up to the idea of regulating cannabis. Regulation could help reduce the risks to health that cannabis use poses, as a regulated cannabis market would introduce some quality control.

This would provide users with information about the strength of cannabis on offer, something they usually only discover after exposure in the current unregulated market. Later this year, scientists at the Institute of Psychiatry will carry out research to find the optimal balance between tetrahydrocannabinol (THC) and cannabidiol (CBD) so that advice to users can be given on how to minimise the potential for developing mental health problems.

Global commission call for legalisation of drugs

However, the greatest risk for most people who use cannabis continues to be exposure to tobacco. In the UK, we are uniquely wedded to the habit of combining tobacco with cannabis in a joint, unlike our American counterparts who ingest cannabis on its own.

For many young people, their introduction to tobacco is via a cannabis joint, increasing the chance of developing tobacco dependence. Regulation could help to break this habit, as information about such risks would be less hampered by the constraints of a substance that falls under the Misuse of Drugs Act.

Ultimately, although the public health message about the link between cannabis and psychosis has been a difficult one to communicate, the clearest advice we can offer with the greatest potential benefit is to leave out the tobacco from your joint.