ANN ARBOR—Although marijuana smoke is known to contain cancer-causing agents, and many studies in humans, animals and cell cultures have indicated that marijuana smoking may predispose a person to cancer, a large epidemiologic study of cancer patients and cancer-free controls in Los Angeles has found no clear association between pot-smoking and cancer.

” We didn’t find any evidence for an increased risk of head and neck or lung cancers among heavy, long-term marijuana users,” said study leader Hal Morgenstern, chair of epidemiology at the University of Michigan School of Public Health. ” I wouldn