Dr Alex Wodak, director of the Alcohol and Drug Service at Sydney's St Vincent's Hospital, says cannabis will soon be Australia's smoke of choice.
"In a few years time, we'll have more Australians smoking cannabis than we have smoking tobacco and by default that market is largely taken over by criminals," Dr Wodak said.
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"Having a black market of that size is not good for anybody and inevitably big black markets can only survive if there's significant police corruption."
Dr Wodak delivered the keynote address at the Australian Drug Law and a Civil Society symposium at the Lismore campus of Southern Cross University on Thursday.
He also heads the Australian Drug Law Reform Foundation.
Drug trade run by criminals
"At the moment, we have no control over cannabis at all because the trade is run by criminals," he said from Lismore.
"By taxing and regulating it, we would start to have some influence over the way people use cannabis.
"Overall, the aim should be to try and reduce the harm."
Cannabis prohibition was expensive and ineffective, Dr Wodak said, with surveys showing up to 2.5 million Australians will smoke cannabis in 2010.
"It's easier for most Australians to purchase cannabis than to buy a pizza - it's a readily available substance," he said.
Dr Wodak said legalising cannabis and regulating it could be carried out similar to what happens in the alcohol and tobacco industries.
Warning labels, age restrictions
"We could have warning labels on packets, we could have age restrictions - we could also have help-seeking information if you're trying to cut down or stop," he said.
Dr Wodak said research had shown punishing people for possessing cannabis does not inhibit their desire to keep using the drug.
"We've proved that we've stimulated a huge black market for cannabis in Australia by prohibition," he said.
He quoted polls in the United States showing support for legalising cannabis had climbed from 12 per cent in 1969 to 44 per cent in 2009.
"I think the minute that politicians start to see that 51 per cent of the population is supporting the taxation and regulation of cannabis, they'll take 10 seconds to work out that's what they want too," Dr Wodak said.
He also expects a legal international trade in cannabis to develop one day, but acknowledged making cannabis a legal drug in Australia and overseas will happen incrementally.
