true, 1 joint has the same amount of tar as about 5 cigarettes or so. and yes, both cigarette and mj tar contain carcinogens. but does this mean that its "bad"? what recent research, has showed though is THC disables an enzyme that deactivates those carcinogens in tar. Thus this is where some hypothesize that marijuana can actually prevent lung cancer. On the other hand, nicotine actively promotes this enzyme so this pathway will eventually lead to cancer.

All of this information is on pubmed if your dr doesn't believe you.