> If it's less bad than tobacco, that's as likely to be usage habits as anything.

If by "usage habits" you mean "usual quantities of material smoked and inhaled", then, sure, that's probably a pretty big difference. Now, legality (and, price, which is strongly influenced by legality) plays some role in that, but the inherent nature of the effects of each drug and their concentrations in the material that is smoked also play a pretty big role.

I have heard (from cannabis advocates, so taking it with a very large halite deposit) that cannabis itself has anti-cancer, or at least anti-tumor, activity. If true, it may be that some of the chemicals present in cannabis may alleviate the harm from the carcinogenic chemicals present in all smoke from burnt vegetable matter. If the effect is large enough, cannabis may still have a net benefit, even when smoked. Maybe.

But really, no one should be inhaling the smoke from anything that's on fire. Nor even eating food seared at high temperatures or by open flames. Some of those Maillard, caramelization, pyrolysis, and carbonization reaction products (like acrolein, acrylamide, heterocyclic amines, and polycyclic aromatics) may contribute significantly to flavor, but they are also very bad for your body, and are similar, if not identical, to the chemicals produced in smoked plants.

So you don't even escape that risk with cannabis edibles, because as I understand it, the plant matter has to be heated above 100C in order to produce the desired recreational effects, and some (impatient) people accomplish that by grilling it in a skillet, destroying some of the potentially medicinal molecules and introducing carcinogenic ones. This is why you really need someone trained in proper lab technique to test the claims. No two amateur stoners will prepare identical cultivars of cannabis in exactly the same way. The cannabis researcher will isolate variables and devise a repeatable experiment.

It is unfortunate that we have little reliable evidence to confirm or deny the anti-carcinogenic cannabis hypothesis, just because of political interference with the science. Few reputable scientists want to touch the stuff, because the DEA makes it such a pain, and there is also the risk of persecution resulting from politically unpopular findings. It would be really, really embarrassing for the US federal government if anyone were actually able to prove that whole cannabis treatments (rather than just a single purified component, like Marinol) either reduce cancers directly or reduce the negative side effects from proven cancer treatments.

So we can't eat baked or fried vegetables either? There's a huge difference between plant matter that is pyrolised and that which is vaporized at about 200c. It's obvious. When you vaporize it mostly remains behind whereas when you smoke it mostly gets turned into ash, which you inhale.

Cannabis has been shown to have anti-cancer properties in at least two clinical studies that I know of. Not the miracle cure that some cannabis advocates say it is, but certainly worth researching.

The fact that cannabis reduces side effects of cancer treatments is well known and a major reason why it is prescribed in the US.

It's absolutely ridiculous that the DEA continues to insist that it's not a medicine, it's clearly a medicine.