Many think marijuana safer than tobacco because it doesn't contain nicotine, but it does contain more tar and cancer causing agents than tobacco.
"Marijuana contains some of the highest cancer causing substances known. It contains carbon monoxide, carbon dioxide, acetone, benzene, toluene, vinyl chloride, dimethylnitrosamine, methylethylnitrosamine, benz(a)anthracene, benz(a)pyrene, ammonia, hydrogen cyanide, and many more." (Huber, Gary: Pharm.Biochem.Behavior Vol.40. P.630, 1991.)
According to
this website, regular users of marijuana may experience the same problems as tobacco smokers including cough,phlegm, chronic bronchitis, and frequent chest colds. In addition, because marijuana contains toxins and carcinogens marijuana smokers increase their risk of cancer of the head, neck, lungs, and respiratory tract.
This website with the answers to some questions about marijuana may interest you.
Ten Most Common Concerns About Cannabis