The number of young people being treated for the side effects of headshop drugs has dropped significantly.
Studies were carried out by Trinity College both before and after the ban on headshops came into effect.
Researchers found a huge reduction in treatment of the side effects of so called "legal highs" since the ban came into force.
Dr Bobby Smyth, who led the research team, said there is some evidence that some of the sales have moved onto the black market, but numbers are greatly reduced.
"We did find some ongoing use of headshop drugs in the months after they closed - so there was some movement, I guess, into the black market," he said
"But the proportion reporting use dropped by about two thirds. More importantly, the number of young people presenting with very problematic use of headshop drugs dropped from one third of all patients down to zero."