Contrary to previous research findings that have suggested a link, marijuana use does not appear to be associated with an increased risk of developing oral cancer, according to a study led by researchers in the Public Health Sciences Division.
Their findings, the result of the most comprehensive evaluation to date regarding the association between marijuana use and the incidence of oral squamous-cell carcinoma, appear in the June issue of Cancer Research, a publication of the American Association for Cancer Research.
The study, conducted in collaboration with researchers at the University of Illinois Urbana-Champaign and Seattle's Center for Health Studies, Group Health Cooperative, found no association between marijuana use and increased oral-cancer risk, regardless of how long, how much or how often a person has used marijuana. The study also found no increased risk among marijuana users who had other underlying risk factors for oral cancer, such as a history of tobacco use or heavy alcohol use.
"When asking whether any marijuana use puts you at increased risk of oral cancer, our study is pretty solid in saying there's nothing going on there," said Dr. Stephen Schwartz, senior author of the study.
The study involved 407 oral-cancer cases and 615 healthy control subjects from western Washington who had been interviewed in detail about their history of marijuana use, among other lifestyle factors. Participants, both male and female, ranged in age from 18 to 65. The oral-cancer cases were identified through a population-based cancer registry housed at Fred Hutchinson that is part of the National Cancer Institute's Surveillance, Epidemiology and End Results program.
The Fred Hutchinson study counters findings from a smaller investigation, widely publicized in 1999, which suggested that ever-users of marijuana were at more than twice the risk of getting head-and-neck squamous-cell carcinoma as compared to non-users.
"Our study casts a fair bit of doubt on the overall conclusion of the previous study," Schwartz said.
The earlier study had a number of limitations, most importantly the fact that its control, or comparison, group was comprised of individuals who had donated blood at the same hospital where the oral-cancer cases had been treated.
"Blood donors tend to have fewer high-risk habits than the general population," Schwartz said, referring to the fact that they're screened for certain lifestyle factors such as intravenous-drug use as well as for previous infection with HIV and hepatitis. Blood donors might therefore be less likely to have a history of marijuana use, he said. "We felt our study, which used controls selected from the general population, could more accurately determine whether oral-cancer patients were more likely to have used marijuana," he said.
Because the incidence of extensive, long-term marijuana use was so low among the study population — a reflection of the population at large — it is unclear whether extremely heavy use over many years is related to the risk oral cancer, Schwartz said.
National surveys have found that about a quarter of the population has smoked marijuana at some point in their lives, and that about 4 percent of the population, or 6 million people, report occasional use of marijuana.
Habitual use on the rise
While the incidence of casual marijuana use has remained stable during the past decade, habitual use or abuse appears to be on the rise. Two recent, nationally representative surveys by the National Institute on Drug Abuse found habitual marijuana use or dependence has increased more than 20 percent in the past decade among American adults, particularly young minorities and baby boomers. Such use is defined as repeated use of the drug resulting in marijuana-related legal problems or difficulties functioning at work, in school or in social situations.
According to the American Cancer Society, more than 28,000 new cases of oral cavity/pharynx cancer are expected this year; incidence rates are more than twice as high in men as in women. The five-year survival rate for all stages combined is 57 percent.