Marijuana is not remotely bad for your body. It is, in fact, one of the better medicinal drugs around, for just that purpose. Side effects from use include slightly red eyes, dry mouth, and a slightly increased heart rate, none of which is harmful in a healthy human being. Long term effects are hard to document, but the only obvious damage that would come from use is a lack of common sense when rolling a joint. Some people choose to cut it with tobacco, so that it burns better. They then use what's known as a roach instead of the filter - a roach is typically just a short, thin, rolled up piece of card. This does not work as a filter, so any of the damaging chemicals in the tobacco will get through to the body. This is the only obvious damage that can be caused, and is only through improper usage anyway.