The standard definition of anorexia nervosa, known by most people simply as "anorexia," is that it's a psychiatric illness that causes an eating disorder marked by poor perception of body image, low body weight, excessive exercise, purging of food (forced vomiting), voluntary starvation, or the use of diuretics and diet pills. It's common knowledge that around 90% of all anorexics are female.
Since anorexia also includes what, in clinical language, is referred to as "an acute loss of appetite, often associated with psychological factors," it would seem logical that cannabis, or at least cannabinoid drugs would be an effective therapy. Pot is known for giving you the munchies, after all.
Unfortunately, since anorexia is, at heart, a psychological disorder that manifests itself physically, the use of cannabis may not help. Cannabinoids have been shown to have only minimal success at appetite suppression in patients with classic anorexia, despite the fact that the primary ingredient, delta-9 THC, has a well-established history with the successful treatment of cachexia, the appetite loss associated with HIV/AIDS.
There are synthetic THC pills, marketed under the name Marinol, which may be used as appetite stimulants in qualified patients, but doctors could lose their licenses by prescribing these drugs as "off-label" treatments for patients with true anorexia nervosa and not suffering from the severe weight loss typically associated with AIDS or cancer chemotherapy.
Similarly, the use of either medical marijuana or synthetic THC may be contraindicated in Alzheimer's disease patients. According to a 2002 study by Grotenhermen, Russo. Cannabis and Cannabinoids, Pharmacology, Toxicology, and Therapeutic Potential (New York: The Hawthorn Integrative Healing Press), "A positive influence on body weight was also reported in 15 patients with Alzheimer’s disease who were previously refusing food. Surprisingly, THC also decreased the observed severity of disturbed behavior. In patients diagnosed with primary anorexia nervosa there was no measurable cannabinoid effect, presumably because the underlying pathological mechanism is not loss of appetite."
Translation: while the use of cannabis or cannabis-derived drugs improved the appetites of Alzheimer's disease patients, it also made their actual Alzheimer's disease worse, while there was no measureable positive effect in anorexia patients.