Conor Purdon, 25, has smoked pot for about 10 years and has never had a bad experience because of it — until earlier this year, in February, when he smoked a more potent kind of marijuana.

"I actually took a hit of what we call concentrate. So it's high-concentrated THC," he says.

THC, or tetrahydrocannabinol, is the main active ingredient in marijuana that gives you the high.

"I hit one of those, and I'm not sure if I rubbed something in my eye, or what happened, but I had a heavy kind of allergy reaction to my face."

It got worse the next morning.

"Next day I woke up, it really hadn't gone away," he told CBC News. There was "swelling in my eyes, really, really, red."

Conor Purdon's eyes swelled from a cannabis allergy in February. (Conor Purdon)

What Purdon didn't know at the time is that he'd had an allergic reaction to pot.

"The consequences of marijuana allergy can be very significant," says Dr. Gordon Sussman, an allergy specialist in Toronto.

"The symptoms are generally due to inhaling marijuana or touching marijuana. So if you inhale the marijuana, you can have a running [nose] and sneezing and itchy watery eyes and coughing, wheezing and shortness of breath. If you touch marijuana you can have hives and swelling," he says.

Just like ragweed, or birch pollen, cannabis is an allergen, a weed that induces allergies. There's little research on the subject, but a European study last year acknowledged the rise in cannabis allergy and its potential health hazard. "The clinical manifestations of a cannabis allergy can vary from mild to life-threatening reactions, often depending on the route of exposure," it says.

A 2013 study that Sussman contributed to says that "cases have been reported where hypersensitivity and even anaphylactic responses have been associated with marijuana use."

Reactions underreported

Pot allergies have been underreported, probably because of the clandestine nature of the marijuana industry. But experts say they expect that to change, with the impending legalization of pot in Canada.

The industry is already regulated, with Health Canada having licensed about 30 medical marijuana production facilities, exposing a wider group of people to marijuana.

There are storefront dispensaries, too, most not sanctioned by Ottawa, operating in a grey market.

Inhaling or touching marijuana can produce an allergic reaction in some people. (David Donnelly/CBC)

"The other group of patients you might want to recognize are people who work in the industry, people who work with marijuana, because they can be sensitized and have allergies, because they work with it all the time," says Sussman.

If their allergies are serious enough, he tells his patients, they should consider avoiding smoking marijuana or think about switching jobs.

For Purdon, that's a non-starter. He uses marijuana for medical reasons. He suffers from nerve pain from a previous snowboarding accident.

He's also the manager of several pot dispensaries in Toronto that sell medical marijuana. And, he has a master's degree in ethnobotany. The irony isn't lost on him. "It was a sad moment when I realized it," he laughs.

Purdon says he figured out on his own that it was pot that caused his allergic reaction, but that he hasn't had a recurrence.

Sussman says many others who get symptoms don't realize they've developed an allergy to pot or don't voluntarily disclose symptoms to their physicians. But it's important they do, says Sussman, so that doctors can better diagnose the problem.

The allergist is currently working on new research trying to characterize the allergens of marijuana and hopefully develop a standard skin test or a blood test for marijuana sensitivity.