Medically, marijuana is no more harmful than alcohol or tobacco. It contains about 400 more carcinogens than tobacco, but many of those carcinogens are almost eliminated when you use steam to smoke it rather than fire (they have a special bong you can get which will heat using hot water and then you inhale it that way; I read about it.

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Alcohol, in excess, ravages almost every system in your body. In moderation, it actually helps almost every system in your body. The same can be said for marijuana. I read a study that claimed, in moderation, smoking marijuana can actually lower your blood pressure. In excess, like alcohol, it will raise it. So, it’s all a matter of moderation vs excess. Jesus drank wine, but not in excess.
I think the only argument that can be made against marijuana is the illegality involved. If you use it in moderation, it can be beneficial.
However, if you use marijuana, or alcohol that matter, for any reason other than leisure, it’s psychologically harmful. If you’re using it to get rid of your problems, then, well, that’s a problem.
That’s bold portion is very important to remember.
Health Risks/Benefits:
You cannot overdose on marijuana - period. A 160lb person would have to smoke 900 joints in an hour to overdose on THC. Do you know anyone who can smoke that much without passing out before hitting 15? I don’t!
It does not kill brain cells. Marijuana in fact activates certain brain receptors. There is NO evidence that marijuana kills them, and I can provide over 50 Dutch studies proving it. It increases the alpha-wave activity in the brain. Guess what increase in such activity causes? An increase in creativity! So, if you’re a writer or painter, there you go.
Marijuana does impair memory, but only during intoxication. 12 weeks after users quit, the short-term memory loss typically disappears.
Marijuana does not affect your heart longterm. However, the use of marijuana by people suffering from certain medical problems can be likened to that same person exercising in excess. So, if you have heart trouble, talk with your doctor before lighting up.
It does affect the immune system. In fact, it acts in the same way cortisone acts - as an immunosuppressent. However, they recommend it for some AIDS patients to increase their weight.
And this is getting long, so I’ll stop. Basically, there are SOME health problems associated with marijuana, but very few and none of them especially harmful to healthy people. I think the biggest problem with marijuana is the risk of cancer, which, like I said, can be lessened (not reduced completely) with certain devices. No device exists, however, which can deliver pure THC via the lungs.
I really don’t believe it is a sin in moderation, despite what anyone says.