Dr Des Corrigan, chairman of the National Advisory Committee on Drugs (NACD), said people mainly got their cannabis from friends and family and mainly used it in friends’ houses.
“I’d say to parents, their children’s friends are crucial. Do you know who your children’s friends are, who the children’s parents are?” he said.
He said children network with each other but parents of children less so, but needed to. He added: “The free gaff is a disaster waiting to happen.”
Dr Corrigan said data from the NACD national survey in 2007 showed that 74% of 15-34 year olds said they were given cannabis by family or friends or were shared among friends.
Some 58% of 15-34 year olds said they last used cannabis in the house of friends.
The NACD chairman gave the opening address at a conference on cannabis – Chilling out or Numbing out? – organised by the Dublin North East Drug Task Force and the Ballymun Local Drug Task Force.
Dr Bobby Smyth of the Drug Treatment Centre Board said parents needed to be “more vigilant”.
“Teenagers have more mobility, more freedom, more money than we did when we were their age, but no more wisdom than we had when 14,” he said.
Dr Corrigan said there had been a “movement” in recent years from weaker cannabis resin to stronger, and more damaging, herbal cannabis.