Marijuana worsens breathing problems in current smokers with chronic obstructive pulmonary disease (COPD), according to a study presented at the American Thoracic Society 2007 International Conference, on Tuesday, May 22.

The study found that among people 40 and older, smokers were two-and-a-half times as likely as nonsmokers to develop COPD, while smoking cigarettes and marijuana together boosted the odds of developing COPD to three-and-a-half times the risk of someone who did not smoke either cigarettes or marijuana - in other words, adding marijuana smoking to cigarette smoking increased the risk by one-third, says Wan Tan, M.D., of St. Paul’s Hospital in Vancouver, British Columbia.

The odds of cigarette smokers having any respiratory symptoms was 2.36 times that of nonsmokers, while the odds of someone who smoked both cigarettes and marijuana having respiratory symptoms was 18 times that of someone who smoked neither—an eightfold jump in risk, Dr. Tan says.

“This study suggests an interaction between cigarettes and marijuana smoking. These findings have not been reported before, and they have a big public health implication,” Dr. Tan says.

A majority of cigarette smokers in the study were also marijuana smokers. In both younger and older adults in the study, 30% smoked both cigarettes and marijuana. Among younger cigarette smokers, 76% also smoked marijuana, while 58% of older cigarette smokers also smoked marijuana.

The findings come from a study of 648 adults ages 18 and older who answered questions on smoking, including their cigarette and marijuana use, and respiratory symptoms. Study subjects ages 40 and older had lung function tests.

The Vancouver researchers decided to study both marijuana and cigarette smoking because both cigarette and marijuana smoking is prevalent in their area, says Dr. Tan. They found that 49% of participants ages 18 to 39 and 46% of those 40 and older had smoked marijuana at least once. Among 18-39 year-olds, 17% said they currently smoked marijuana, compared with 13% in the 40+ age group.

In the younger group, 31% said they had ever smoked cigarettes, and 16% were current smokers. In the 40+ group, 52% were ever-smokers while 16% were current smokers.

“The Impact of Cigarette and Marijuana Smoking in Chronic Obstructive Lung Disease Study in Vancouver, Canada” (Session C38; Abstract # 681; Poster Board # L42)

Source: American Thoracic Society (ATS)