Legalising the medical use of cannabis has not led to a surge in the numbers of adolescents using it in the USA, according to new research that surprised its authors and will encourage those hoping for relaxation of the law elsewhere.

Since 1996, 23 US states and the District of Columbia (DC) have approved the medicinal use of cannabis. In the states of Colorado, Washington, Alaska and Oregon and DC, recreational use is also legal. These moves towards permissiveness, even where possession of the drug is restricted to medical use, have caused many critics to worry that cannabis use would rise, especially among teenagers.

That assumption was the starting point for the research carried out by Dr Deborah Hasin, professor of epidemiology at Columbia University Medical Center in New York, and her colleagues. However, the findings from 24 years of data from more than one million adolescents in the 48 contiguous states did not substantiate those fears. Their paper in the journal Lancet Psychiatry says that the use of cannabis by adolescents was already higher in the states that have opted for medical legalisation. But the change in the law did not lead to a jump in numbers.

Analysing data from a national study called Monitoring the Future, which collects information from 50,000 pupils aged 13 to 18 in the 8th, 10th and 12th grade (years 9, 11 and 13 in Britain) every year, they found there had not been a rise even after taking into account individual, school and state-level factors that can affect marijuana use (such as age, ethnicity, public or private school and proportion of each state’s population that was male or white).

“Our findings provide the strongest evidence to date that marijuana use by teenagers does not increase after a state legalises medical marijuana,” said Dr Hasin. “Rather, up to now, in the states that passed medical marijuana laws, adolescent marijuana use was already higher than in other states.”

That still gave cause for concern, she suggested: “Because early adolescent use of marijuana can lead to many long-term harmful outcomes, identifying the factors that actually play a role in adolescent use should be a high research priority.”

Among the youngest students surveyed, the 8th graders, marijuana use actually dropped. The authors speculate that the older students’ views on the drug may already have been fixed before medical legalisation, but that those who were younger were less likely to view it as recreational once medicinal use was authorised. Another possibility is that parents were taking a more vigilant and robust stance against it, the authors said, adding that this warranted further investigation.

In a commentary in the journal, Dr Kevin Hill, from the division of alcohol and drug abuse at McLean Hospital in Belmont, Massachusetts, writes: “Perhaps the main concern of many people opposed to medical marijuana laws is that they will lead to increased general marijuana use, including among adolescents.”

Cannabis use has increased in the US among teenagers in recent years, unlike nicotine, alcohol or opioids, and the perception of its dangers has dropped, he points out. “Hasin and colleagues postulated, as many would, that the passage of medical marijuana laws would increase adolescent marijuana use by contributing to the declining perception of the potential harms of marijuana. Their well-designed, methodologically sound study showed that this was not the case,” he says.

The study shows how important it is to test out hypotheses with rigorous research before making health policy, he says. “The growing body of research that includes this study suggests that medical marijuana laws do not increase adolescent use, and future decisions that states make about whether or not to enact medical marijuana laws should be at least partly guided by this evidence.”

David Nutt, professor of neuropsychopharmacology at Imperial College London, said the result of the study was as he would have predicted. “The illegal market for cannabis is probably saturated, so making it a medicine will just allow law-abiding citizens with chronic medical problems to obtain the relief ‎they have been denied for the past 40 years of prohibition,” he said.

British experts campaigning to reschedule cannabis in the UK to allow medicinal and research use welcomed the study. “Patients are suffering unnecessarily and others in great pain are travelling abroad to find the cannabis they need to ease their symptoms,” said Prof Val Curran of University College London, the UK’s leading expert on medicinal cannabis and joint author of a new report for the all-party parliamentary group for drug policy reform. “All this could change by moving cannabis from schedule 1 to schedule 2, thus recognising the medicinal value of the drug.”