The legal status of cannabis puts a ''massive block'' between client and clinician, Pact mental health clinical leader Matthew Peppercorn told a cannabis public forum at Dunedin Hospital.
Clients feared getting in trouble if they discussed cannabis, eliminating opportunities to minimise harm from its use.
There were some people who should never use it, he said.
Mr Peppercorn believed it was a ''travesty'' synthetic cannabis products would be approved and regulated under the Psychoactive Substances Act, instead of using the natural version as safely as possible.
The new regime seemed like a big money-making venture.
High-profile media coverage created ''shenanigans'' around K2 and other products, which only made people want to try them, he said.
Community Alcohol and Drug Service medical director Dr Gavin Cape, who favoured cannabis law reform, said like many things in life, it had a harmful and addictive side.
Those with a family history of mental illness should be wary of using it, as should adolescents. Smoking it posed respiratory and cardiovascular risks, he said.
There was a lack of research into cannabis, and studies had shown both positive and negative effects.
University of Otago anthropologist Geoff Noller, who has researched its use, said he recently attended a drug forum in Wellington where there was concern over the Government's lack of response to the 2011 Law Commission review of drug law, which recommended taking steps towards legalising cannabis for medicinal use.
Dr Noller felt ''pessimistic'' about the prospect of law reform as politicians shied away from it. This was despite clear evidence of medicinal cannabis' efficacy.
Otago Norml spokesman Abe Gray said Invermay could be transformed into a hemp research centre. Hemp was an untapped opportunity for economic development.
After the forum, Southern District Health Board candidate Julian Crawford, of the Aotearoa Legalise Cannabis Party, said he would push for the availability of medicinal cannabis if elected in October.
The forum was organised by Pact.
