Both cancer itself and the various treatments for it cause several side effects, including anorexia and cachexia, as well as chemotherapy-induced nausea and vomiting. Others experience severe pain and difficulty sleeping. For many of those patients, one substance is prescribed to help get them through it: marijuana.
Anorexia and Cachexia
Anorexia, the loss of the desire to eat (or the loss of appetite), is a common symptom for cancer patients though it may occur earlier or later in the disease. Cachexia which sometimes comes with the anorexia (but not always) is a wasting condition where patients have significant weakness and a progressive loss of weight, fat, and muscle. Cachexia sometimes occurs in patients who are eating, but aren't absorbing nutrients correctly.
In these cases, marijuana, and specifically THC, either used alone or in combination with nutrients provided by tube helps improve appetite. It's basically a medical use of a common pot smokers' side effect: the "munchies."
Nausea and Vomiting
Anti-cancer drugs sometimes cause nausea and vomiting because they're designed to affect the part of the brain which controls those responses, or because they irritate the lining of the stomach. There are several factors that affect the severity of these symptoms, including which drug is being used, the dosage, the schedule, and the patient's reaction.
Because cannabis has anti-emetic (anti-nausea) properties, it's extremely useful for treating cancer patients with severe nausea. Since it can be smoked or vaporized, as well as give in a sublingual (under the tongue) tincture, it can be used even with patients who can't eat. It can even be mixed with liquid nutrition offered via feeding tube.
Pain Relief and Sleep Aid
Marijuana isn't technically an analgesic, but it does help relieve pain by offering tense muscles. Some patients have said it takes away pain, but it would be more accurate to say that it takes away the awareness of pain. Its soporific effects, which recreational smokers might refer to as "couch lock," also help patients get restful sleep.
While no official statement has been made, new research from several sources points to the possibility that cannabis, or some of the components therein, may actually arrest cancer, stopping the growth of cancer cells and tumors. Most of this research is being done in Europe, because U.S. regulations make it difficult for even researchers to test cannabis.