Massachusetts voters in 2½ weeks will consider becom­ing the 17th state to ­legalize the use of marijuana for medical purposes. But there is little research showing whether the drug has therapeutic benefits.

--------------------------------------------------

Picture: Ethan Ruby smoked from a vaporizer packed with a small quantity of marijuana at his Denver home.

That says more about the difficulty of studying an illegal substance than it does about the inherent medical value of the plant.

Opponents of the proposal say medical practice should be defined by rigorous study and drugs vetted by the US Food and Drug Administration, not by popular opinion. Advocates say federal drug policy has thwarted that process. They point to abundant anecdotes and a collection of small studies that found marijuana to be effec­tive in alleviating pain and muscle stiffness in people with chronic conditions.