First off let me start by saying I've never tried marijuana in my life.
As a doctor, I get this question alot: "Cigarettes or marijuana, which is worse?". Well, to be totally frank about it, the adverse effects of tobacco both on general health and in pregnancy have been documented time and time again in the medical literature. Marijuana, on the other hand, does in fact contain certain toxic chemicals, but nowhere near approaches the numbers of cigarettes. It does in fact cause (what psyche docs call) "chronic ambivalence" in the medical literature, as well as muscle relaxation (why people with MS use it) and increased appetite (why some cancer/AIDS patients use it).
As far as drugs go, cannabis is by far not the most harmful one-UNLESS IT IS MIXED WITH OTHER HARMFUL AGENTS-here in CT I see alot of kids mixing with PCP and even Embalming fluid to potentiate the high. This is when marijuana becomes lethal. I believe if marijuana were legalized and regulated, at least some governing body could control what goes into the drug. There are very few cases of overdosing on cannabis alone, but this number gets much bigger with mixing of marijuana and other drugs. My view: legalize it, regulate it, tax it heavily, use the extra money to fund anti-drug education initiatives. Just my $0.02.