The measure by Assemblyman Ian Calderon (D-Whittier), dubbed the "right-to-try" act by supporters, would have authorized drug manufacturers to let patients facing imminent death obtain drugs that are still undergoing review by the Food and Drug Administration.
The FDA does allow for such patients to apply for "compassionate use" access to medications that are still in clinical trials, but proponents of the "right-to-try" legislation say the application can take too long for the gravely ill.
Brown, in his veto message of AB 159, noted the FDA recently streamlined the "compassionate use" program.
"Before authorizing an alternative state pathway, we should give this federal expedited process a chance to work," Brown wrote.
This is the second time in one week in which Brown has weighed legislation meant to address the suffering of Californians with terminal illnesses. On Monday, he approved a high-profile measure that will allow dying patients to legally obtain medication to end their lives.