A lot of the science is out on this one.
On one hand most weed smokers tend to smoke less frequently than tobacco smokers.
On the other hand, smoking a single joint is generally found to be a good deal more damaging to your respiratory system than smoking a single cigarette (no filter, for one), so it's not necessarily any healthier.
I can say with some degree of personal experience that you smoke weed regularly it can have quite an adverse effect on your life and health. If nothing else, I don't know anyone who smoked regularly who hasn't had some kind of psychotic reaction at some point.
However, smoking the very occasional joint, and giving your body and mind time to recover in between probably won't do you a great deal of harm. Smoking a joint at a party once a month is very different from smoking one every day or every week.
I'd never advocate smoking weed. I've seen it properly fuck up a lot of people close to me, and no amount of hearing people who enjoy it advocate its benefits can erase that for me. However, if you have the information I don't see why you shouldn't make an informed choice, and I don't think occasional/social smoking is a big enough problem to warrant the level of stress it's clearly causing you. If you want to do it, just make sure you're not doing it all the time.
Kukulski said:
My grandfather died because Alzheimer's disease ate half of his brain. Chill out about the joint, man.
3 of my grandparents did that. However, they were about 70 or 80 when they did.
My other grandfather died of lung cancer caused by smoking. He was about 50 at the time.
20 years off your life is still a long time.