SEATTLE (AP) — Marijuana has been shown to help ease pain and a few other health problems, yet two-thirds of U.S. states have decided pot should be legal to treat many other conditions with little scientific backing.

At least 1.4 million Americans are using marijuana for their health, according to an Associated Press analysis of states that track medical marijuana patients.

The number of medical marijuana cardholders more than tripled in the last five years as more states jumped on the bandwagon. The analysis is based on data from 26 states and the District of Columbia. The total climbs to 2.6...