New York is on track to become the 21st state to legalize medical marijuana this year, and two states—Colorado and Washington—have decriminalized recreational use as well. Americans now overwhelmingly support fewer restrictions on marijuana, with 86 percent saying doctors should be allowed to prescribe the drug for medical purposes.

Despite its surging popularity, the jury is still out on whether marijuana is truly the panacea its supporters claim it to be. Until recently, the drug's illegal status impeded rigorous study of its effectiveness. Several research groups are now taking advantage of today's looser laws to seek out answers. Here is where we stand for the six most studied conditions.

Cancer

Numerous trials have indicated that medical marijuana increases appetite and reduces chemotherapy-related nausea in the short term. Yet it may not be as effective as other recently developed drugs, so marijuana is not considered a first-line treatment for these symptoms.

Epilepsy

Multiple animal studies have suggested that THC, one of the main psychoactive chemicals in cannabis, may inhibit the brain processes thought to cause seizures. High-quality human studies are lacking, however, leaving many open questions.

Glaucoma

Several studies have found that smoking marijuana lowers pressure inside the eye, relieving glaucoma-related discomfort for about three to four hours. Yet a number of pharmaceutical drugs have been shown to be more effective and longer lasting than medical marijuana.

HIV/AIDS

In one randomized controlled trial, patients given a cannabislike compound were twice as likely to gain weight as patients given a placebo—a boon for people battling the wasting effects of this disease. The treatment's long-term effectiveness remains untested.

Multiple sclerosis (MS)

A large trial published in 2012 found that a cannabis extract significantly decreased muscle stiffness and other MS symptoms. A smaller study found that smoking cannabis worked better than a placebo in reducing both spasticity and pain in treatment-resistant participants. Given the few therapies available for MS, a 2011 review concluded that medical marijuana might be a viable way to manage certain symptoms.

Pain and inflammation

Studies have suggested that marijuana is only slightly better than a placebo in reducing acute inflammation, and it may even increase the perception of pain in some patients. When taken in combination with other medications, however, various cannabis-derived drugs have been shown to be moderately effective for reducing chronic neuropathic pain.

Medical Marijuana's Deep Roots

2700 B.C.:

Legendary Chinese emperor Shennong is said to discover the healing properties of marijuana.

1500 B.C.:

Medical papyri from Egypt mention marijuana as a cure for sore eyes and inflammation.

600 B.C.:

Bhang, a drink of cannabis and milk, is used widely as an anesthetic in India.

A.D. 79:

Roman naturalist Pliny the Elder cites boiled cannabis roots as a treatment for gout, pain and cramped joints.

A.D. 800:

Islamic physicians prescribe cannabis for a variety of ailments, although some decry it as a “lethal poison.”

A.D. 1542:

German physician Leonhart Fuchs names the plant Cannabis sativa.

A.D. 1842:

British army surgeon William Brooke O’Shaughnessy introduces marijuana into medical practice as a treatment for pain, nausea and convulsions.

A.D. 1850:

The United States Pharmacopeia, which issues the official handbook for dispensing medications, classifies marijuana as a legitimate medical compound.

A.D. 1925:

A League of Nations treaty limits cannabis use to medical or scientific purposes. Aspirin and other newly developed drugs begin to replace cannabis as treatments for pain.

A.D. 1930:

Harry J. Anslinger is appointed commissioner of the Federal Bureau of Narcotics. He considers marijuana a corrupting influence and helps to turn public opinion against the drug.

Happier Days

Although clear evidence for medical marijuana is lacking for many ailments, it is important to note that these clinical results do not consider improvements in overall quality of life. Many users report improved mood and decreased anxiety and insomnia as invaluable benefits of the drug. Furthermore, some of the pharmaceutical treatments that have outperformed marijuana come with unpleasant side effects. Consider Megestrol, a drug frequently given to people with cancer or AIDS to increase appetite. It encourages more sustainable weight gain than marijuana does but has the distressing side effect of impotence. For taxing conditions with few treatments, such as MS, medical marijuana may be the only way to provide relief for patients who have exhausted conventional therapies. So although most doctors do not recommend medical marijuana as a first-line treatment, they often prescribe it either in combination with other medications or as an alternative for patients with a low tolerance for side effects.