International experts and community activists have condemned the government's decision to reclassify cannabis.
Cannabis was reclassified to a class C drug earlier this year
More than 300 people gathered at Belfast City Hall on Thursday to debate the issue.
The government reclassified cannabis to a class C drug in January, putting it on a par with tranquilisers.
The Advisory Council on the Misuse of Drugs said it was less harmful than other class B substances.
However the body concluded the drug was not harmless and posed risks to health and society.
Thousands of posters have been sent out to schools and colleges across the Northern Ireland, warning that cannabis is still illegal, despite the reclassification which came into effect last month.
Keith Hellawell, the government's former drugs tsar, was the keynote speaker at the conference.
He believes reclassification is going to have far reaching consequences for Northern Ireland.
"You are late in the game and I felt there was a great opportunity to understand and learn from what has happened in England, in particular, and bring those lessons here to prevent a repetition of what has happened on the mainland," he said.
Two weeks ago, the head of the police drugs squad warned that paramilitaries and organised criminal gangs were using the proceeds of cannabis and ecstasy to try to break into the hard drugs market.