The use of marijuana as a medicine is remarkably uncontroversial at the bedside of a cancer patient or a child suffering from convulsions who might be helped. As this consensus has grown into broad public acceptance, many state legislatures have leapfrogged federal prohibition and the Federal Drug Administration process to allow medical use of cannabis. But they have usually failed to translate compassion into safe, effective health care. Until recently, when New York got it right.

The Compassionate Care Act, passed by the...