Lighting up a cigarette has known cancer-causing consequences, but marijuana’s link to cancer has been less understood. Now, after a meta-analysis of 25 studies, scientists say that smoking marijuana heavily for at least a decade heightens the risk of one startling disease: testicular cancer.

Co-author Deborah Korenstein, a physician at Memorial Sloan Kettering Cancer Center, tells Inverse that, importantly, this risk is “not huge.”

However, she does emphasize that men who currently use marijuana should be aware that regular use probably does increase this risk. Regular use in the United States is not uncommon: Approximately 13 percent of Americans say they “regularly” smoke, and this rate is higher among people between the ages of 18 and 29.

The weed-cancer connection

The systematic literature review spans fifty years of data and was published in the journal JAMA. The review explored smoking marijuana’s potential links to dozens of types of cancer, including lung, head, neck, and urogenital cancers.

The researchers discovered that heavy marijuana use (a daily joint for ten years) was associated with the development of testicular germ cell tumors (TGCT), which make up 95 percent of all testicular cancers. Heavy weed smokers had a 36 percent higher chance of developing TGCT than non-weed smokers, the study shows.

Study links long-term smoking with a heightened risk for testicular cancer. Dank Depot/Flickr Creative Commons

Korenstein explains that, presently, her team doesn’t know how these results are influenced by dose or duration. One joint a day for ten years could amount to the same as five joints per day for four years. This will have to be determined in subsequent studies.

TGCT is the most common cancer in men between ages 20 and 40, and affects about 2 percent of all men. TGCT can grow quickly: Often, men notice a small painless bump on their testicle or change in ball size. They can look for signs of TGCT using a self-exam.

Other researchers have connected heavy cannabis use with higher incidence of testicular cancer. But in this review, the evidence for increased risk of TGCT wasn’t strong. More research is needed to confirm just how risky smoking weed is for testicular health.

What’s in a puff?

Weed smoke shares some dangerous similarities to cigarette smoke. A puff of weed and tobacco both contain carcinogens, substances capable of causing cancer in living tissue. In fact, some research shows toxic gases, such as benzo[a]pyrene and phenols, are 20 times higher in unfiltered marijuana than cigarette smoke.

Because people tend to take larger puffs, hold their breath longer, and inhale more deeply when they smoke weed versus cigarettes, they can have higher exposure to harmful properties like tar and carbon monoxide.

Because of this dangerous overlap in the properties of cigarette and marijuana smoke, an uptick in lung cancer seems likely from heavy weed smoking. The review suggests heavy marijuana smoking may spike lung cancer, but the evidence was mixed.

The properties of marijuana smoke and cigarette smoke may be more similar than people think, research shows. Jamie Grill/Getty Images

Scarce research meant the review’s authors couldn’t draw conclusions from the data about how regular marijuana use impacts the development of other types of cancer. Some research shows cannabinoids, compounds found in cannabis plants, inhibit proliferation of cancer cells. More longitudinal studies are needed to determine the relative risks and benefits linking marijuana and cancer.

However, just because the team didn’t find strong evidence showing an increased risk of other types of cancer doesn’t mean weed smokers are totally in the clear, Korenstein says. She notes that the limitations of the data preclude us from drawing firm conclusions.

"I’d hate for people to interpret that to mean that marijuana use is completely safe.”

“I’d hate for people to interpret that to mean that marijuana use is completely safe,” Korenstein says. “The fact is that we don’t know much about the impact of heavy use, use at a very young age, or about non-smoking use.”

Young smokers beware

Importantly, researchers may have seen the higher rates of testicular cancer because most weed smokers are in fact young men, and often, clinical studies recruit young, university-age people to enroll in studies.

Korenstein says that, while young smoker habits need to be further evaluated, it’s possible that they are at a higher risk.

“Testicular cancer tends to be a disease of younger men, so heavy marijuana use at an early age could be particularly risk,” she says. “We just don’t know.”

There are a number of unknowns: There’s not enough data on women or the elderly to understand how their marijuana habits might lead to cancer, and the researcher aren’t sure how the drug delivery method — whether someone smokes, eats, or vapes weed — influences cancer rates.

Korenstain cautions that, because of this bulk of unknowns, people should understand that this area of study is still one of uncertainty — and it could be possible that harmful evidence linked to smoking marijuana may not emerge for many years.

Partial Abstract:

Importance: Marijuana use is common and growing in the United States amid a trend toward legalization. Exposure to tobacco smoke is a well-described preventable cause of many cancers; the association of marijuana use with the development of cancer is not clear.

Data sources: A search of PubMed, Embase, PsycINFO, MEDLINE, and the Cochrane Library was conducted on June 11, 2018, and updated on April 30, 2019. A systematic review and meta-analysis of studies published from January 1, 1973, to April 30, 2019, and references of included studies were performed, with data analyzed from January 2 through October 4, 2019.

Main outcomes and measures: Rates of cancer in marijuana users, with ever use defined as at least 1 joint-year exposure (equivalent to 1 joint per day for 1 year), compared with nonusers. Meta-analysis was conducted if there were at least 2 studies of the same design addressing the same cancer without high risk of bias when heterogeneity was low to moderate for the following 4 cancers: lung, head and neck squamous cell carcinoma, oral squamous cell carcinoma, and testicular germ cell tumor (TGCT), with comparisons expressed as odds ratios (ORs) with 95% CIs.