Here's a new review in the Yale Journal of Biology and Medicine.
Cannabinoids (drugs produced by pharmaceutical companies) have been reported effective for appetite stimulation, analgesia and relief of nausea. The effects are modest and other currently available drugs appear to be more effective. Evidence is lacking for other conditions.
Cannibas (e.g. marijuana plant products) has not been validated for medical use in systematic research, although there are anecdotal reports including dramatic testimonials.
Doctors face a conundrum in states that allow medical cannabis. According to the review:
Medical marijuana schemes create problems for prescribers. Laws allowing physicians to prescribe cannabis conflict with U.S. federal law, which does not allow the use of cannabis for any purpose. Under the U.S. Constitution, federal laws pre-empt state laws [4,48]. The Bush administration threatened to strip doctors of their licenses to practice if they recommended marijuana to their patients. Even when the U.S. courts removed this threat, physicians remained reluctant to recommend cannabis because of concerns that they would be legally liable for any harms experienced by their patients [47,51]. In the absence of data, physicians also found it difficult to decide to whom they should recommend cannabis, in what amounts, and for how long [54,55]. These challenges have been ignored by a small number of physicians, who advertised their preparedness to provide patients with a medical recommendation for a fee.
Medical cannabis initiatives may deliver little more than de facto legalization of recreational marijuana.