The study, the first to link marijuana with such cancers, suggests that the drug's popularity in recent decades could have serious long-term health consequences for some users.

The generation of Americans who were teenagers in the 1960s, when recreational use of marijuana became widespread, is reaching the age when many types of cancer start to become more common.

Marijuana smoke is higher in tar and carcinogens than tobacco smoke, and previous research has shown that marijuana smokers, like cigarette smokers, can develop precancerous changes in cells lining the respiratory tract. Researchers said they therefore were not surprised at the news that smoking marijuana predisposes users to head and neck cancers, and they predicted it will likely be found to increase the risk of lung cancer as well.

"It's what I expected to see," said Li Mao, an associate professor of medicine at M.D. Anderson Cancer Center in Houston, who has followed research in the field. "It appears marijuana [smoke] is a stronger carcinogen than cigarette smoke."

Nevertheless, the independent effect of cigarette smoking on an individual's cancer risk is probably greater than that of smoking marijuana, noted Eugenia Calle of the American Cancer Society (ACS), "because people just smoke so many more cigarettes."

Researchers have estimated that 31 percent of Americans who were over 11 years old in 1992 have tried marijuana. However, Mao said, few studies have examined marijuana's impact on long-term cancer risk. He added that because the drug is illegal, it is more difficult to get people to be honest about their past marijuana use than about their cigarette-smoking history.

"It probably will be much harder than [with] cigarette smoking to prove" a link between marijuana and lung cancer, Mao said.

Cancers of the mouth, throat or larynx occur in 40,400 Americans annually and cause 12,300 deaths. Cancers in these areas are often curable if caught early. Smoking (of cigarettes, pipes or cigars), chewing or sniffing tobacco, and drinking alcohol are proven risk factors.

Lung cancer is the number one cause of cancer mortality, striking 171,600 Americans and killing 158,900 annually. Tobacco smoking is estimated to cause 80 percent of all lung cancers. Asbestos, radon and occupational exposure to certain carcinogens are also recognized risk factors.

Researchers decided to investigate the possible role of marijuana in head and neck cancers because marijuana smoke is high in tar and people who use the drug tend to inhale deeply, depositing four times more particulate matter in the mouth, throat and windpipe than cigarette smokers, said Zuo-Feng Zhang, a professor of epidemiology at the University of California at Los Angeles School of Medicine and principal author of the new study. "If marijuana smoking has anything to do with smoking-related cancer, the oral cavity would be the first site being affected," he said.

For the study, performed while Zhang was working at New York's Memorial Sloan-Kettering Cancer Center, researchers enrolled 173 patients with head or neck cancer as well as a comparison group of 176 cancer-free blood donors of similar age and sex. All study participants were questioned about their past use of marijuana, tobacco and alcohol, workplace and environmental exposure to possible carcinogens, and other aspects of their background and lifestyle. Frequency of marijuana use was categorized as never, less than or equal to once per day, and more than once per day. Duration of use was categorized as never, one to five years and more than five years.

Among those who had ever used marijuana, the risk of head and neck cancer was 2.6 times greater than among those who had never used the drug. (Researchers arrived at this figure after adjusting for the effect of other risk factors, such as smoking and alcohol.) They also observed a dose-response effect of marijuana, with heavier users at higher cancer risk. Among people who reported smoking marijuana once per day, the risk of head and neck cancer was 2.1 times that of someone who never used it, while among those who reported smoking it more than once per day, the risk was 4.9 times that of those who had abstained.

Furthermore, people who were current smokers of both tobacco and marijuana had by far the highest risk of head and neck cancer, indicating that the two substances work together synergistically to promote cancer development. Current users of both substances had 36 times the risk of head and neck cancer found in people who used neither.

Calle, who is director of analytic epidemiology at ACS, said the new findings underscore the long-term danger of smoking marijuana and the need for research into better ways of delivering the drug's active ingredients if it is to be used by people with chronic illnesses to relieve pain, nausea, muscle spasms and other symptoms, as some have advocated. "ACS is supportive of research into the benefits of cannabinoids [marijuana's active ingredients]," she said, "but . . . they do not advocate the use of inhaled marijuana."

Should current or former marijuana smokers have special checkups? Considering the new findings, Calle said, they might be wise to follow current ACS guidelines on head and neck cancer for tobacco smokers. The society suggests that smokers stay on the lookout for any unusual sores, lumps or patches on lips or in the mouth, tongue, cheek or neck; ask their dentist or dental hygienist to check the mouth and tongue closely at each visit; and consider having a doctor examine the mouth and neck area annually or twice a year.

Head and Neck Cancers at a Glance

FREQUENCY

Cancers of the mouth, tongue and throat: 29,800 new cases per year; 8,100 deaths.

Cancer of the larynx: 10,600 new cases per year; 4,200 deaths.

SIGNS AND SYMPTOMS

Mouth or throat cancer: A sore in the mouth that does not heal (most common symptom); a lump or thickening in the cheek; a white or red patch on gums, tongue, tonsil or lining of mouth; sore throat or feeling that something is caught in throat; difficulty chewing, swallowing or moving jaw or tongue; numbness in the mouth; swelling of jaw; loosening of teeth; jaw or tooth pain; voice changes; a lump in the neck.

Cancer of the larynx: Hoarseness lasting more than two weeks; lasting cough or sore throat; difficulty or pain with swallowing; ear pain; trouble breathing; a lump in the neck.

RISK FACTORS

Tobacco use and alcohol consumption (especially heavy drinking) are risk factors for both mouth cancer and cancer of the larynx. Poor nutrition, a weakened immune system and infection with certain human papilloma viruses (the family of viruses that cause warts) have also been implicated in both. A new study suggests current or past marijuana smoking is also a risk factor.

For mouth cancer, additional risk factors include sun exposure (to lips) and possibly high-alcohol mouthwash and poorly fitting dentures.

For cancer of the larynx, frequent or severe heartburn (reflux of stomach acid into the throat) is a risk factor.

TREATMENT

Varies depending on tumor stage. Many of these cancers can be cured if caught early.

SOURCE: American Cancer Society Web Site:

http://www.cancer.org/

By Susan Okie
Washington Post Staff Writer
Tuesday, January 11, 2000; Page Z06
© Copyright 2000 The Washington Post Company

Related Article:

A Downer for Aging Marijuana Users - 12/28/99
http://www.cannabisnews.com/news/thread4118.shtml

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