THC, THE ACTIVE INGREDIENT in marijuana that makes you "high", is a helpful drug for certain diseases, and it may help treat lung cancer -- see http://www.sciencedaily.com/releases/2007/04/070417193338.htm. In addition, it helps relieve SYMPTOMS of diseases or side-effects of certain treatments. For instance, for cancer patients, it helps calm their nerves, helps control pain to a certain degree, and helps settle their stomach and reduce vomiting; but most of all, it makes them "hungry". One horrible side effect of chemo is the total loss of appetite, which decreases nutrition and adds to weight loss, which is the LAST thing a cancer patient needs. Marijuana or THC, has been shown to help in the above problems. In fact, any disease or treatment that makes someone not want to eat, and that becomes in important issue, in many cases THC will increase ones appetite. THC has been shown, in some cases, to slow the advancement of glaucoma. Also, with people with AIDS who are too skinny, are helped to feel hungry and fight off the rapid weight loss.
Cancer is an abnormal growth in the body, cells affected do not follow the normal growth signals for the particular cell, called apoptosis, according to http://www.drugpolicycentral.com/bot/pg/cancer/THC_cancer_nov_2003.htm THC actually helps in the process getting this signal to tumors so they stop dividing and die.