Cannabis smoke can seem gentler than tobacco's, but that impression of marijuana may be deceptive. Deep in the lungs, the drug's chemistry is far from benign, recent research suggests. Some studies have even linked heavy cannabis use to a heightened risk of some cancers, including those of the head and neck.
So is there a clear link between cannabis and cancer, and if so, what causes it?
Each lungful of cannabis smoke is a brew of the high-inducing ingredient tetrahydrocannabinol (THC), fine particulates, and certain carcinogens that are also found in tobacco. But cannabinoids may bring their own twist by quietly rewiring the lungs' immune settings.
"The airway's protective lining weakens, antiviral signals falter, and inflammation flares," said Raphael Cuomo, a scientist working at the intersection of cancer prevention and addiction at the University of California, San Diego. In one study with mice, cannabis smoke dulled both tumor-fighting and infection defenses. In another, THC blunted the effects of immunotherapy, weakening tumor-killing immune cells in the body.
Cannabis also triggers other changes in the immune system, according to studies in humans, cells and lab mice. In the airways, levels of proteins tied to inflammation spike when cannabis is used consistently. A protein called MUC5AC, which lines and protects the respiratory tract, is made in excess in certain cells, hinting at early tissue changes. Cannabis smoke can skew the enzymes that maintain tissue and switch off genes that respond to toxins.
The responses of the lungs' patrol cells — alveolar macrophages — can be stunted. And in the blood, shifts in cellular messengers called cytokines and faint changes in epigenetics become a molecular diary of exposure.
Related: Could cannabis treat cancer someday? Here's what the science says so far
In summary, "cannabis smoke, from its toxic byproducts to THC, can irritate lungs and weaken immunity," Sayantan Bhattacharyya, a cancer systems biologist who will soon be a senior postdoctoral scientist at the University of Limerick in Ireland, told Live Science.
But immune disruption is only one factor that might raise cancer risk. Cannabis smoke may also tug on deeper levers inside cells — the molecular switches that cancers use to grow and spread.
Among these, few are as infamous as the epidermal growth factor receptor, or EGFR. In one of the few studies of its kind, Bhattacharyya examined 83 men with glottic cancer, which affects the voice box, and found that cannabis smokers in the group had markedly higher activation of EGFR and related cancer proteins than either tobacco smokers or nonsmokers.
"EGFR governs [cell] growth, repair, and survival," Bhattacharyya said. "In cannabis smokers, constant activation can fuel unchecked proliferation, speed genetic damage, and hasten the leap toward cancer — triggering earlier onset, faster spread, and treatment resistance." That said, Bhattacharyya's study included only 83 people with one type of cancer, so more work is needed to know if the results are broadly applicable.
Bhattacharyya sees two priorities for future research. One is to develop realistic lab models — such as "organoids," 3D-printed tumors and organ chips — to study how cannabis amplifies EGFR. The other is to probe potentially dangerous additives that might be lurking in unregulated cannabis products, he said.
For all the unsettling biology uncovered in laboratory studies, real-world studies of cannabis smokers tell a mixed story. Some research links heavy cannabis use to higher cancer risk, while others find no clear connection. Still, Cuomo maintains that long-term use of the drug may increase the odds of developing cancer, based on what we know so far.
For people who already have cancer, however, Cuomo has shown that cannabis use isn't linked to an earlier death — a puzzle researchers now call "Cuomo's paradox." In fact, studies have found that, in cancer patients, cannabis can boost appetite and curb chemotherapy-induced nausea, even as its smoke may potentially be planting the chemical and cellular seeds of disease.
The challenge going forward will be to accurately assess both the benefits and harms of cannabis in order to weigh them against one another.
Editor's note: This story was updated on Aug. 21, 2025, to note that Bhattacharyya will hold a position at University of Limerick.
This article is for informational purposes only and is not meant to offer medical advice.
