"You have to treat 24 patients to find one patient who has a 30 percent or more reduction in their pain, 23 out of 24 patients won't even get a 30 percent reduction in their pain," he said.
Dr Alchin said other medications for chronic pain had been proven to be much better than cannabinoids.
However, he said scientific reviews had also showed it could be very effective for paediatric epilepsy, pain and spasticity in multiple sclerosis, or for those with nausea from chemotherapy.
"It's not a blanket solution for everything but that's the way it's being presented - it's the new 'miracle drug' - but that's not what the data shows," he said.
He said this week's British Medical Journal detailed the disappointment from patients that some forms of cannabinoids, made prescribable, were not being prescribed.
"Patients have been up in arms because doctors haven't been prescribing it, but the reason doctors haven't been prescribing it is because they look up the guidelines and the conditions [are] ... not actually indicated for."
He said doctors were being caught in the crossfire.
GP Buzz Burrell, who has an interest in pain management, said medicinal cannabis was an excellent debate to have but it was a premature time to be having it.
Mr Burrell was concerned patients would be disappointed with results, as well as the long term implications of medicinal cannabis products which - other than for MS and epilepsy in childhood - had not officially gone through phases of testing.
"To ask a prescribing community to prescribe a product that hasn't really gone through phase one trialling is incredible," he said.
"I can't think of any drug which I've been asked to prescribe in my 32 years of practice which hasn't at least gone through phase-four trialling."
He said there were lessons to be learned from drugs such as fenfluramine, which despite helping with obesity was then found to increase the need for valve open heart surgery.
"I would hate to think I started deliberately prescribing a cannabis product only to find 10 years from now that there's a significant increase in incidents where rare diseases were happening," he said.
Dr Burrell said he was concerned people were blurring the lines between recreational and medicinal cannabis, and felt if people wanted to do grow their own that was fine but doctors should not be involved with it.
"In the same way it would be ridiculous to ask medical practitioners to start prescribing whisky because people feel better when they take it, to prescribe home-grown cannabis is pushing us back at least 150 years in the world of medicine," he said.
- RNZ
