Skip to main content

The Globe and Mail

Dosage, THC levels and other points to consider before trying medical marijuana

Nearly matured medical marijuana plants grow in a climate controlled growing room.

James MacDonald/Bloomberg


My mother had a mastectomy after being diagnosed with breast cancer. It's now six months since her operation and she is still suffering from sharp, burning pain where her breast was removed. The drugs she has been given for the pain aren't providing enough relief. I've heard that marijuana is good for pain. Should she ask her doctor for a prescription for pot?


Story continues below advertisement

It's true that some studies, involving relatively small groups of patients, suggest marijuana can help ease certain types of pain. But using marijuana as a medicine isn't necessarily easy because there aren't yet standardized doses and treatment protocols.

Under the existing federal rules for medical marijuana, doctors are expected to assess patients and provide them with a document – essentially a prescription – stating how much marijuana they should consume per day. The patient is then supposed to get a supply of marijuana from one of 43 licensed producers who sell their products online.

Many doctors aren't sure what dose to recommend because of a lack of guidelines that are based on reliable scientific evidence, says Dr. Hance Clarke, director of the pain-research unit at the Toronto General Hospital.

To further complicate matters, not all marijuana is the same.

Clarke points out that most prescription drugs contain a specific dose of a single active compound.

In contrast, marijuana plants have hundreds of different chemical compounds, including cannabinoids, which affect a wide range of processes in the human body.

At least two cannabinoids are thought to have therapeutic benefits – cannabidiol (CBD) and tetrahydrocannabinol (THC), the ingredient that makes people feel high.

Story continues below advertisement

The amount of CBD and THC varies from one strain of marijuana to the next. And there's no data regarding the ideal ratio of CBD to THC for treating pain.

Despite these uncertainties, Clarke says more and more patients are asking for marijuana, which is increasingly seen as a panacea for their medical ills. However, some patients have ended up in hospital emergency departments because they were unprepared for the mind-altering effects of the drug.

Before they try real cannabis, Clarke says marijuana novices should be prescribed a low dose of synthetic THC – such as nabilone, a capsule originally approved for treating severe nausea and vomiting in people undergoing cancer chemotherapy.

Even with experienced marijuana users, Clarke says physicians need to provide them with guidance on what to do.

"I don't prescribe anybody marijuana unless they do some homework," Clarke says. "They have to look at the websites of the licensed producers and understand the different strains of marijuana and the different levels of THC and CBD they contain."

He cautions that patients should be getting marijuana only from licensed producers – and not storefront dispensaries where the source of the cannabis may be unknown.

Story continues below advertisement

For the best results, patients should closely monitor their reactions to marijuana and adjust the dose and strain accordingly, Clarke says.

Other physicians, who regularly prescribe marijuana, agree that it's important for patients to play an active role in determining the most effective dose for themselves.

"You should start with a low dose and increase it to see if your pain control is improving," says Dr. Arsenio Avila, an anesthesiologist and pain specialist at Sunnybrook Health Sciences Centre in Toronto.

He says patients should not expect their pain to be eliminated by marijuana alone. "It's mostly used in combination with other drugs."

But what's encouraging is that marijuana appears to make it possible for some patients to get by on lower doses of other pain medications, including opioids.

Avila notes that patients tend to develop a tolerance to opioids. That means they need higher and higher doses of an opioid to get the same level of pain relief. The higher doses, in turn, carry increased risk of side effects including drowsiness, nausea and severe constipation.

"With marijuana, you start to get a better response to your existing drugs – and you can start to reduce them," Avila says.

What's more, marijuana seems to be especially useful in relieving neuropathic or nerve pain – an extremely hard-to-treat condition that sometimes occurs after certain operations, such as mastectomies, amputations and thoracotomy (chest) surgeries.

For some reason, the nerves in the area of the surgery become altered or damaged and they lose their ability to transmit normal sensations. A simple touch on the skin may produce a stabbing, burning or electric-shock feeling.

Marijuana, working on the central nervous system, may help to dampen these unpleasant sensations.

Although some of the initial research is promising, much larger studies are needed to confirm both the benefits and risks of medical pot, Clarke says.

Ideally, these questions should be answered before cannabis becomes widely available next year when the federal government plans to legalize recreational marijuana.

But that's unlikely to happen, Clarke says. There are no financial incentives for licensed producers to invest in this type of research. They can easily sell their products in the absence of proper dosing guidelines.

"I think marijuana has an important role to play in several areas of medicine," Clarke says. "We just need the science to back it up."

Paul Taylor is a Patient Navigation Advisor at Sunnybrook Health Sciences Centre. He is a former Health Editor of The Globe and Mail. You can find him on Twitter @epaultaylor and online at Sunnybrook's Your Health Matters.

Report an error
As of December 20, 2017, we have temporarily removed commenting from our articles as we switch to a new provider. We are behind schedule, but we are still working hard to bring you a new commenting system as soon as possible. If you are looking to give feedback on our new site, please send it along to If you want to write a letter to the editor, please forward to