A new study has found no evidence that legalising marijuana for medical use does not increase the rate of teenagers using the drug

Legalising marijuana does not encourage more young people to use the drug, scientists have today concluded.

A new study of more than one million adolescents in the US has found no evidence of a link between making the drug available for medical use, and increased use in teenagers.

Covering 48 states in the US, the study analysed data over 24 years from 1991 to 2014 - both before and after laws legalising cannabis.

Since 1996, 23 states and the District of Columbia have passed legislation allowing medical use of the drug.

But the move has provoked concern that such legislation will result in more teenagers trying and regularly using cannabis.

However, researchers at the Columbia University Medical Center found 'no significant difference' in adolescent use in 21 states.

Dr Deborah Hasin, Professor of epidemiology at the university in New York, said: 'Our findings provide the strongest evidence to date that marijuana use by teenagers does not increase after a state legalises medical marijuana.

'Rather, up to now, in the states that passed medical marijuana laws, adolescent marijuana use was already higher than in other states.

'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.'

Researchers examined the relationship between legalisation of medical marijuana and teenage use of the drug, but analysing the national Monitoring The Future survey.

The survey covers more than one million students in grades eight, 10 and 12, aged 13 to 18, between 1991 and 2014.

During that time 21 contiguous states passed laws allowing the use of cannabis for medical purposes.

Their findings revealed that although marijuana use in the previous 30 days was more prevalent in states that enacted medical marijuana laws than those that did not, rates of teenage use did not increase after the legislation was passed.

The evidence held up, even after scientists took into account other factors that could affect teenagers' use of the drug, including age, ethnicity, and whether they attended public or private school.

Since 1996, 23 US states and the District of Columbia have passed legislation allowing medical use of the drug

Dr Kevin Hill, from the Division of Alcohol and Drug Abuse, at McLean Hospital in Belmont, said: '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.

'The framework of using a scientific method to challenge what might be ideological beliefs must remain an important driver of future research on marijuana policy.'