Do we know that? Smoking cigarettes is awful and uppers will kill you (another reason cigs are bad!), but outside of that, I don't think you are right.
Now, most of my (non marijuana) drug use is on that upper tip, but still. Also, it would be silly to think that one thing doesn't effect the other. Currently I run at least 4 miles a day. I eat a clif bar and blueberries for breakfast, a banana, then broccoli sprouts and raspberries, then an apple midday and just whatever for dinner (usually a pretty healthy thing)...if I don't get lung cancer or emphysema I probably won't have super negative effects from smoking (yer heart diseases, etc.). I could definitely get cancer though.
In cigarette smokers, smoking marijuana decreases lung cancer. Opiates are some of the healthiest things you can put into yourself, as far as damage to your body, as long as you don't overdose. Hallucinogenics have little to no effect on your body. What are other drugs? Mephedrone? That shit was the best thing I have ever done, but I've never felt worse the next day, so probably not great? Ketamine and PCP probably aren't awesome to do... I don't know man, I think some drugs are bad and some are neutral and some are good, just like every other fuck*ng thing? But yeah, if you leave your comment at cigarettes you are probably right?
After all, the only reason we are here is cause some Apes found Psilocybin cause they got forced out of the forests.
Edit: Yes...drinking, that is the other awful drug. I don't do that one. IDK, pick your battles, do what you like the most (I love smoking cigarettes and being high off of marijuana) and try to balance that out with other things?
Maybe it is where I live/lived (BOULDER), but there are a shit load of yuppie, holier than thou vegans...sorry.