Cancer Study Headline, Translated: Cannabis Smoking and Cancer Risk

The headline and the study underneath it

Cannabis and cancer headlines swing hard in both directions. One week smoking it causes cancer. The next week it does not, or it even fights cancer. Both stories are usually built on the same shaky base.

Almost all of this evidence is observational. That means researchers watched groups of people rather than assigning anyone to smoke or not smoke. Observational studies can show a link. They struggle to show a cause.

The National Cancer Institute's cannabis summary opens its risk section with a single honest sentence. A number of studies have yielded conflicting evidence about the risks of various cancers linked to cannabis smoking.

What the studies actually found

It helps to see the spread.

• A pooled analysis of three case-cohort studies of men in northwestern Africa covered 430 cases and 778 controls. It found a significantly higher risk of lung cancer among tobacco smokers who also inhaled cannabis.
• A large retrospective cohort study of 64,855 US men aged 15 to 49 found no link between cannabis use and tobacco-related cancers. Among men who did not smoke tobacco, ever having used cannabis was linked to higher prostate cancer risk.
• A case-control study of 611 people with lung cancer found that low long-term cannabis use was not linked to lung cancer. It found no positive link with oral, pharyngeal, laryngeal, lung, or esophageal cancer. That held once cigarette smoking and other factors were taken into account.
• A systematic review of 19 studies of lung lesions found no significant link between cannabis inhalation and lung cancer. That was after adjusting for tobacco use.
• The National Academies of Sciences, Engineering, and Medicine reviewed the published meta-analyses. It concluded there was moderate evidence of no statistical link between cannabis smoking and lung cancer.

Head and neck cancer is messier still. A pooled analysis of nine case-control studies covered 1,921 oropharyngeal cancer cases. It also covered 356 tongue cancer cases and 7,639 controls. Cannabis smokers had higher risk of oropharyngeal cancer. They had lower risk of tongue cancer. A separate review of nine case-control studies, with 13,931 people, found the evidence insufficient either way.

Testicular cancer has its own thread. Three population-based case-control studies linked cannabis use to higher risk of testicular germ cell tumors. A Swedish study followed 49,343 men conscripted at ages 19 to 21 for 42 years. It found no link with ever using cannabis. It did find that heavy use, meaning more than 50 times in a lifetime, was tied to a 2.5-fold higher risk.

Why the answers keep changing

Four problems sit under nearly every one of these studies.

The first is tobacco. Many people who smoke cannabis also smoke cigarettes. NIDA notes that high rates of tobacco use among cannabis users make it hard to separate the two. A study that does not adjust well for tobacco will blame the wrong smoke.

The second is dose. Most studies ask whether a person ever used, not how much or for how long. Ever is a weak measure. The Swedish study is instructive because heavy use behaved differently from any use.

The third is recall. People are asked to remember habits from decades ago, often after a cancer diagnosis has already changed how they think about their past.

The fourth is the product itself. THC concentration and delivery methods have changed a great deal. Studies of people who smoked joints in 1970 may not describe someone vaping concentrate today.

What is not in dispute

Two things hold up regardless of which cancer study you read.

NIDA states that cannabis smoke contains many of the same toxins, irritants, and carcinogens as tobacco smoke, and that smoking it can harm lung tissue. Long-term cannabis smoking is linked to airway inflammation, higher airway resistance, lung hyperinflation, and chronic bronchitis.

Cannabis also raises heart rate and blood pressure right after use, and some research ties long-term use to higher stroke and heart attack risk.

So the honest summary is narrow. The cancer signal is unclear. The lung and heart signals are not.

When to get checked, and for what

Screening first. A yearly low-dose CT scan is advised for adults aged 50 through 80 with a 20 pack-year tobacco history. They must smoke now, or have quit within the past 15 years. Cannabis use does not qualify a person for that program. No screening test exists for cannabis smoking alone. Our page on lung cancer screening covers who is eligible.

Symptoms matter more here. Contact a clinician for:

• A cough lasting more than three weeks, or a cough that changes.
• Coughing up blood, even a small amount, even once.
• Shortness of breath or chest pain that is new.
• Hoarseness lasting more than three weeks.
• A sore in the mouth or on the tongue that has not healed in three weeks.
• A lump in the neck that lasts more than three weeks.
• Trouble or pain swallowing.
• Weight loss you did not intend.

Those last four point toward head and neck cancer, where human papillomavirus and tobacco are the dominant risk factors. The first four point toward lung cancer.

The change that actually moves risk

Whatever a cannabis headline says next, the largest single thing a person who smokes can do for cancer risk is stop smoking tobacco. That link is not conflicted, not observational-only, and not waiting on more data. Our guides to tobacco and cancer and quitting smoking cover what works.

Reading the next one

When the next study lands, four questions sort it quickly. Was it observational or randomized? Did it adjust for tobacco use? Did it measure dose, or only ever versus never? And how many cancers of that type did it actually contain? A study with 356 cases of one cancer cannot settle much on its own.

Sources

• NCI PDQ, Cannabis and Cannabinoids (Health Professional Version) — https://www.cancer.gov/about-cancer/treatment/cam/hp/cannabis-pdq
• NCI PDQ, Cannabis and Cannabinoids (Patient Version) — https://www.cancer.gov/about-cancer/treatment/cam/patient/cannabis-pdq
• NIDA, Cannabis (Marijuana) DrugFacts — https://nida.nih.gov/publications/drugfacts/cannabis-marijuana
• USPSTF, Lung Cancer: Screening — https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/lung-cancer-screening