That’s unfortunate, because as more jurisdictions decriminalize or even fully legalize the drug, its relationship with lung cancer is unfurling as an important public health matter. Knowing that, the Canadian Medical Association says this:

“Chronic [marijuana] users often have shortness of breath after exercise, coughing and chest tightness. It is probably associated with bronchitis and emphysema and may have risks for chronic lung disease and lung cancer, comparable to cigarette smoking. This is less of a problem for those that use vaporizers, as a harm reduction strategy.”

The science community has struggled to wrangle enough people together for a study who: a) are willing to be honest about how much they smoke, when in many jurisdictions it’s either criminalized, or, at best, still taboo; b) represent a wide range of marijuana smoking habits, from light to heavy, man. But there’s another big barrier to designing high-quality studies: cigarettes tend to muck up the details. Someone who smokes weed heavily may also smoke cigarettes, making it difficult to say what’s actually responsible if that person got a lung cancer diagnosis.

Canadians are behind some of the latest studies to try to wrestle with this mess. Most recently, researchers at the Lunenfeld-Tanenbaum Research Institute, at Toronto’s Mount Sinai Hospital, published a paper last year in the International Journal of Cancer that analyzes what it purports to be the “largest data set on cannabis and lung cancer risk to date.”

As part of the International Lung Cancer Consortium, they have access to a wealth of global data on lung cancer, and studied the pot-smoking habits of a pooled analysis sample of 2,159 individuals with confirmed lung cancer cases and 2,985 “control” individuals without.

Within that broader group, when tobacco was pulled from the equation, researchers found “little evidence” for increased risk of lung cancer among “habitual” pot smokers (who had smoked at least one “joint-year,” or 365 joints over their life) and long-term pot smokers. This is in line with previous, smaller studies. Still, one should caution that the sample size, with tobacco smokers removed, limited the reach of this study; in fact, the researchers behind it declined to comment on the results because of its limitations.

Another messy issue is that the way people smoke pot is different than the way they smoke cigarettes and other tobacco products. Pot smoke is usually inhaled longer and deeper, and often without a filter – which may lend itself to being considered more dangerous than cigarette smoke.

But consider this: If a heavy smoker goes through two packs a day, that’s 40 or 50 cigarettes. If someone’s a self-declared pothead, that might mean three, five, maybe 10 joints a day. (Or bowls, bong rips, vapes, whatever the kids are hauling on these days. Joints, at about 0.75 grams, are the unit of choice in a lot of the literature.)

No matter how you grind it – or smoke it – the amount of smoke in the lungs for marijuana smokers is far less than for heavy tobacco smokers. All of this may – or may not – make marijuana’s relationship with lung cancer different than tobacco’s.

There are hints that heavy usage may come with higher risk. Dr. Russ Callaghan, an associate professor at the University of Northern British Columbia in Prince George, B.C., published a research paper in 2013 that examined the health outcomes of 49,321 Swedish men who were conscripted into the military in 1969 and 1970. His team found that, adjusted for tobacco use, “heavy” marijuana smoking – which, in the study, meant more than 50 times in a lifetime – was associated with a doubled risk of developing lung cancer over the 40-year follow-up period. Risk, of course, is not causation, especially since other lifestyle factors come into play.

Dr. Callaghan conducts long-term, population-based studies that relate to public health. “I think it’s important that the public should know about the harms that come with cannabis use, especially smoking,” he said in a phone interview.

His study looked at a huge number of people over a vast amount of time, but he, too, cautions of the limitations of his study: “The sample of heavy marijuana users who aren’t tobacco users,” he said, “is quite small.” He cites the Lunenfeld-Tanenbaum study as “one of the best in the field – better than mine,” but cautions that “consensus is still out” on any direct link between pot and lung cancer.

As jurisdictions make access to pot easier and in some cases legal, the public’s demand to know more will only increase; research on the connection is likely going to continue for a long, long time.