One of the most frightening symptoms of advanced cancer is "cachexia", or severe, unintentional weight-loss and wasting. It's a terrible prognostic sign, and the only truly effective treatment is removal of the cancer. Treatment of this syndrome has the potential to improve quality of life in patients with advanced cancers. Various types of medications, including antidepressants, hormones, and cannabis derivatives have been tried with little effect. Treating the symptoms of incurable cancers is difficult and although we're pretty good at it, we sometimes fail. Cannabis seems a plausible intervention, given the anecdotal and clinical data associating it with increased appetite, although appetite in normal, healthy individuals may be mediated by different pathways than the cachexia in cancer patients. Still, it's worthy of investigation.
(As an aside, what a person with advanced cancer does to find relief is their own business. I hope that we don't fail them so miserably that they have to resort to desperate measures. I once had an elderly patient who was shooting up heroin for his cancer pain because he didn't understand the medical system well enough to seek proper help. He did fine on long-acting oxycodone.)
The Journal of Clinical Oncology published an interesting study in 2006 investigating the possible use of cannabis to treat cancer-related cachexia. Since self-administration of self-procured pot is rather inexact, the investigators compared whole marijuana extracts, purified THC, and placebo. The results were a disappointment for those looking for better treatment of advanced cancer.
While cannabis extract was well-tolerated, there was no difference between the groups, although all groups including placebo had some relief of symptoms. Appetite and quality of life saw similar changes in each group. It may be argued that the dose was insufficient, but the investigators chose the dose based on previous studies that showed too many adverse effects at higher doses.
As already mentioned, the failure could be due to a number of reasons, the most likely being that cannabis does not have a mechanism of action that acts on the same pathways as cachexia---the inflammatory cytokines present in cancer cachexia do not seem to be significantly modulated by cannabis. While cannabis may make some people feel better, there is no consistent evidence that it is any better than placebo in making patients with cancer wasting syndrome feel better.
Strasser, F. (2006). Comparison of Orally Administered Cannabis Extract and Delta-9-Tetrahydrocannabinol in Treating Patients With Cancer-Related Anorexia-Cachexia Syndrome: A Multicenter, Phase III, Randomized, Double-Blind, Placebo-Controlled Clinical Trial From the Cannabi Journal of Clinical Oncology, 24 (21), 3394-3400 DOI: 10.1200/JCO.2005.05.1847