Teen marijuana use doesn't increase after laws allow the drug's medical use, according to a new study.

Legalising medical marijuana doesn't increase cannabis use among adolescents.

New research, which analysed data from 1991 to 2014 from more than one million American adolescents, showed no significant difference in the drug's use in 21 states with medical marijuana laws before or after they were introduced.

The authors of the study, published in The Lancet Psychiatry, said such laws were controversial and raised concerns about increased accessibility and acceptability of marijuana to teenagers.

Adolescent use of marijuana is associated with adverse later effects, so identifying factors underlying their use is of substantial public health importance, say the authors.

In Australia, the NSW government on the weekend announced plans to build a $12 million medicinal cannabis research centre as it presses ahead with clinical trials.

And on Friday, it was revealed that Barry and Jo Lambert were donating $33.7 million to the University of Sydney for research into the medicinal applications of cannabis.

The study's leader author, Dr Deborah Hasin from New York's Columbia University Medical Centre, said some commentators suggested the medical marijuana laws sent a message to adolescents that the drug's general use is acceptable.

The findings showed 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 adolescent marijuana use did not increase after these laws were introduced.

"Our findings provide the strongest evidence to date that marijuana use by teenagers does not increase after a state legalises medical marijuana," she said.