The short answer is, "We don't know."  Why not?  Because existing DEA rules make it virtually impossible to carry about appropriate double-blind trials.

Yet the anecdotal evidence for marijuana's efficacy is stunning; here is one good example:

Even though it's a crisp November day, the flower boxes of Mary Jones's neat little bungalow are overflowing with brightly colored blooms. The bubbly mother of three has her utility vehicle parked in the driveway. Her hair is perfectly coiffed, her blond highlights glimmer in the late-fall sun. She looks like she could be a real-estate broker, and seeing the rock on her manicured finger, I imagine for a moment that her husband is a doctor or a lawyer. Mary would, in fact, be the ideal soccer mom, except that one of her now-grown sons played football, and rather than working in real estate, she grows and sells marijuana.

Read the rest here.  Anecdotes do not prove that marijuana works, but they make a good case for allowing objective scientific evaluation.