Donald P. Tashkin, MD, Director of the Pulmonary Function Laboratories at the University of California, Los Angeles, wrote in his Mar. 1997 article for the Center for Substance Abuse Prevention titled "Effects of Marijuana on the Lung and Its Immune Defenses":

“Analysis of the smoke contents of marijuana and tobacco reveals much the same gas phase constituents, including chemicals known to be toxic to respiratory tissue…

With regard to the carcinogenic potential of marijuana, it is noteworthy that the tar phase of marijuana smoke contains many of the same carcinogenic compounds contained in tobacco smoke, including polycyclic aromatic hydrocarbons, such as benz[a]pyrene, which was recently identified as a key factor promoting human lung cancer…

Bronchial immunohistology revealed overexpression of genetic markers of lung tumor progression in smokers of marijuana.

Preliminary findings suggest that marijuana smoke activates cytochrome P4501A1, the enzyme that converts polycyclic hydrocarbons, such as benz[a]pyrene, into active carcinogens.”

Mar. 1997