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Deciding when to prescribe medical marijuana is contentious for Canadian doctors. (Steve Dipaola/Reuters)

Many doctors remain ambivalent about prescribing medical marijuana for chronic pain, even though new federal rules allow patients in Canada to legally purchase dried cannabis with a doctor's prescription, a debate recently highlighted in a medical journal.

Monday's issue of the Canadian Medical Association Journal includes two pro and con commentaries: 

  • New medical marijuana regulations: The coming storm
  • Medicinal cannabis: Time to lighten up?

Dr. Meldon Kahan of the Substance Use Service at Toronto's Women's College Hospital and Dr. Anita Srivastava of St. Joseph's Health Centre and the University of Toronto argue physicians should advise some patients to quit smoking marijuana and refer them to treatment.

Kahan and Srivastava say the only clear indications for medical marijuana are neuropathic pain conditions (a type of chronic pain that affects the nerves) and spasticity from multiple sclerosis. Smoking cannabis is not recommended for common pain conditions such as fibromyalgia or lower back pain, they say.

Patients will ask doctors to prescribe cannabis for chronic pain but physicians need help to understand how to counsel people about the requests.

"Smoked cannabis has short-term and long-term safety risks," the pair wrote. "Smoking cannabis has been associated with an increased risk of motor vehicle crashes, schizophrenia, mood disorders and addiction."

Some of the products in smoked cannabis are carcinogenic, and a recent large long-term retrospective study showed an increased risk of lung cancer, the pair said.

Prescriptions should state that the producer supply a strain with no more than nine per cent THC, the main psychoactive ingredient in cannabis, Kahan and Srivastava said.

For people who smoke marijuana, they suggested doctors advise:

  • Don't light up daily.
  • Don't mix it with alcohol or sedating drugs.
  • Don't drive after smoking.

Dr. David Juurlink, an internist and head of the division of clinical pharmacology at the University of Toronto, said doctors shouldn't accede to every request for medical cannabis as if it were an "innocuous panacea" because it is not.

But doctors also shouldn't reject cannabis as a medicine out of hand just because it makes them feel uncomfortable, he said. Concerns about pot's safety and effectiveness shouldn't deter doctors from prescribing it in some cases, especially when opioid drugs like oxycodone carry heavier baggage in terms of toxicity.

"I'm trying to discourage physicians from the perception that drugs obtained at a pharmacy are necessarily better for an individual patient than cannabis, if a patient has reported benefits from its use," Juurlink said.

Juurlink's motivation was partly a patient he saw in hospital with a long history of anxiety and a litany of psychotropic medications who almost took his own life in response to the addition of a new drug.

When Juurlink spoke to the man, he said the only time he felt relief was when he smoked a joint in the evening while walking the dog. Juurlink said the story resonated with him and prompted him to suggest that the man talk to his family doctor about medical marijuana.

In cases such as this man's, Juurlink advocates for doctors to make decisions about prescribing cannabis when it is sometimes deemed the right thing to do, just as every other prescription decision is made after balancing the risks, benefits and patient-specific factors.

Marijuana can also be inhaled through a vaporizer, which a hospital in Sherbrooke, Que., recently approved.

Two costly pharmaceutical synthetic cannabinoids, the pill nabilone and the spray nabiximol, are licensed for use in Canada and avoid the respiratory effects of smoking marijuana.

With files from CBC's Amina Zafar