A licence to use cannabis legally has been proposed by leading expert on the drug Professor Roger Pertwee.
Professor Pertwee said making cannabis as available as alcohol would prevent drug-related crime, and reduce the chances of people being introduced to harder narcotics.
But he cautioned that it might be necessary to prevent vulnerable individuals obtaining the drug.
"You'd need to have a minimum age of 21, and I would suggest you might even have to have a licence," said Prof Pertwee, from the University of Aberdeen, who pioneered early research on the effects of cannabis in the 1960s and 1970s.
"You have a car licence and a dog licence; why not a cannabis licence?"
The idea would mean only those not suffering from a serious mental illness or at risk of psychosis would be legally allowed to buy the drug.
Research has shown an association between smoking cannabis and a greater chance of some individuals developing schizophrenia.
Professor Pertwee said cannabis appeared to increase the risk of psychosis in people already predisposed to the illness because of their genes or traumatic childhood.
He called for a greater debate on the recreational use of cannabis, and said in principle he was in favour of legalisation, if the right framework could be found.
"We need to explore all the various options," said Prof Pertwee, who is speaking at the British Festival of Science at Aston University, Birmingham, this week.