Does smoking marijuana increase the risk of lung cancer? The jury’s still out

Though marijuana use is now legal in most of the United States, questions remain about how smoking marijuana can affect lung health and the risk of lung cancer in particular.

Research has suggested that smoking marijuana can cause damage to the lungs but is not as detrimental as smoking tobacco.2-4 Studies have also indicated that smoking marijuana may not increase a person’s risk of lung cancer, but robust evidence is lacking.2-9

“We concluded that there is moderate evidence that there is no association between cannabis smoking and lung cancer,” said Donald I. Abrams, MD, professor emeritus of medicine at the University of California San Francisco and integrative oncologist at the USCF Osher Center for Integrative Health. 

Dr Abrams is one of the experts who reviewed evidence on the health effects of cannabis for a report published in 2017 by the National Academies of Sciences, Engineering, and Medicine (NASEM).

Among the research reviewed for that report was a pooled analysis of 6 case-control studies encompassing 2159 individuals with lung cancer and 2985 control participants. The analysis showed “little or no association” between the intensity, duration, cumulative consumption, or age at the start of cannabis smoking and the risk of lung cancer. 

Another study reviewed for the NASEM report was a population-based study of cannabis use in 49,321 Swedish military conscripts. Researchers evaluated the risk of lung cancer over a 40-year follow-up period. They found a 2-fold increase in lung cancer risk among participants with a history of “heavy” cannabis use (defined as having smoked more than 50 times at baseline) compared with those who had no history of cannabis use. 

However, this study was limited by a lack of data on tobacco use. The researchers did adjust for tobacco use at baseline, but they did not examine whether participants smoked tobacco at any point during the follow-up period. In several other studies linking cannabis smoking to an elevated risk for lung cancer, participants were also tobacco smokers.

Reasons for insufficient evidence

Studies published since the NASEM report was released have concluded that current evidence is insufficient to confirm associations between marijuana smoking and lung cancer. These include a 2019 meta-analysis of 25 studies8 and a 2021 meta-analysis of 34 studies9 exploring the link between marijuana and various cancers. 

The lack of evidence stems, in part, from challenges associated with conducting well-designed studies, according to Charles A. Powell, MD, professor of medicine and system division chief for pulmonary, critical care, and sleep medicine at the Icahn School of Medicine at Mount Sinai in New York, New York. 

“The strong evidence base linking cigarette smoking and cigar smoking to an increased risk of lung cancer took a long time to develop,” Dr Powell said.

“It is more challenging to establish similarly robust scientific evidence with epidemiological studies of marijuana smoking.” 

Along with the difficulty of acquiring precise data on a drug that has been illegal in the United States until recently, many “crucial endpoints take a long time to develop, and this is important for the relatively recent onset of increased marijuana smoking,” Dr Powell said. 

In addition, although the composition and carcinogen content of cigarettes are generally standardized, it is relatively difficult to measure the health impact of marijuana smoking because of the variable product content and modes of delivery. 

Other effects of smoking marijuana

Dr Powell noted that there is evidence demonstrating stress and injury to the lungs after exposure to any smoked product, aside from the issue of lung cancer risk.

“Lung cells have strong capabilities to respond to injury, and the outcomes include full repair, cell death, airway inflammation, lung fibrosis, permanent changes to the cell DNA, and cancer,” Dr Powell explained.

“These short-term and long-term responses can occur when smoking cigarettes, E-cigarettes, marijuana, cigars, and water pipes.”

Various factors determine an individual’s response to these exposures, according to Dr Powell. Susceptibility to lung injury and lung disease is determined, in part, by genetics, but there is a lack of data to enable prediction of which smokers will develop cancer. 

“My advice is not to smoke, period,” Dr Powell said. “There are alternative approaches to access marijuana other than through smoking, and there is no evidence to date—nor is there likely to be—that oral or topical use of marijuana products will injure the lungs.”

In the context of medical marijuana use for symptom relief in patients with cancer,10 however, Dr Abrams recommended that “if patients want better control over the onset, depth, and duration of the effect, inhaling is probably better than ingestion.”

“Further research is indicated to better understand the impact of different marijuana cigarette products and different approaches to smoke marijuana,” Dr Powell said. In addition to being useful for health care providers, “this information may be useful to those who cannot or will not quit,” he said.