Originally Answered: Is the drug "weed"bad?
There's the truth, what the government tells you, and what potheads will tell you. I've done quite a bit of research on the subject and here it is.
1. It's addictive in the same sense that chocolate is addictive. If you've eaten chocolate before and you liked it, you'll probably want to eat it if it's around. That said, if you don't want to eat it it's not too hard to resist if you have a good amount of willpower. I smoke weed occasionally myself and I'd say that's a pretty good analogy.
2. Inhaling carbon is never good for your lungs. It's not too bad either if you don't smoke often because you're lungs heal themselves and flush out crap like tar. It's been said that the damage is parallel to that of tobacco smoke, which is only true in a sense. If you grew the tobacco in a well-ventilated room, it's damage would be the same as weeds. But research from various prestigious universities and orgs have recently shown that tobacco leaf fibers strongly attract Radon and Polonium particles, which commonly in reside in very small amounts in both soil and indoor air. Thus, tobacco smoke in realistic settings is in fact worse than marijuana smoke in that that of tobacco contains radioactive particles that build up in the alveoli and are, in fact, the main cause of lung cancer due to smoking tobacco. So weed is bad for your lungs, but not really that bad in moderation.
Weed makes you hungry, and so you tend to eat more than you should when your high. If you employ willpower and exercise, this shouldn't be too much of a problem. Abusing weed tends to lead towards lethargy, which isn't great for your physical health. An effect of weed is hightened blood pressure, which is bad for those with cardiovascular conditions.
Weed is terrific for stress relief in the long term (due to it's effects, but I won't go into specifics), which leads to a healthier state of mind. It has also been shown to reduce the risk for Alzheimers, Parkinsons, and other mental diseases, but I'm not 100% convinced since too few studies have been done on this subject. The only danger it poses to mental health is that it can, in rare instances, exacerbate psychoses and bring genetically predisposed psychoses to surface. Basically, if you're family has a history of mental illness, you shouldn't use weed or any other psychedelics.
All in all, except for a few unlucky people, weed is very safe with responsible usage and is, in my opinion, very beneficial for your mental health.
3. Unless the guy pushes you to smoke after you've told him you don't want to -- which would probably create awkward tension --, you should definitely still be friends with him.