I am familiar with the research on alcohol. I wouldn't call it an unmitigated plus, but there are enough offsetting positives that I can rationalize my fondness for spending time with "Al."
Yeah, I didn't mean to say that either was an unmitigated plus. Very few things in life are.
Is there research out there showing positive health effects from MJ?
You might have heard of medical marijuana
Granted, I think a lot of it is just "backdoor" legalization, and it seems to be the case that in medical marijuana states it is possible to easily obtain a prescription from doctors (presumably because they, who should best know, realize what nonsense prohibition is).
I would guess that a lot of the positives would be offset by lung damage
One hears a lot about that - perhaps even that the smoke, gram for gram, is more harmful than tobacco smoke. But smoking 20 or more cigs a day is typical. I think very few pot smokers consume anywhere near that many joints - plus a joint is typically less herbage than a cigarette, plus joints are usually shared, plus a lot of people are using devices that "clean" the smoke somewhat (e.g. water pipes and vaporizers). And of course, if you or your doctor decide you're getting too much of that smoke, with MJ you're not dealing with a drug that's arguably more addictive than heroin.
Anecdotally, in graduate school I did research on new methods of detecting lung disease. Our methods were able to detect very distinct differences between a young healthy non-smoker and a young apparently-healthy cigarette smoker; in fact, data from the latter looked more like that from an elderly person crippled by COPD. Back then, when I was getting my PhD, I smoked a lot more pot than I do now (but wait, I thought pot was supposed to kill ambition ?!?). Even so, running the tests on myself, I looked more like a non-smoker than an asymptomatic tobacco smoker.
As usual, insight can be gained by following the money. (As one of the cops on "The Wire" says "You follow drugs, you get drug addicts and drug dealers. But you start to follow the money, and you don't know where the **** it's gonna take you !") I'm sure far more money has been spent trying to show that marijuana is bad for you, than to show it's good for you. With tobacco, the opposite is the case (at least if you count the lawyers trying to litigate that there's not proof it's bad for you). Even with this bias, look where we are.