Two Sacramento County men who died when they were shot with stun guns both were taking medication for mental illness.
The deaths renew concerns about the supposedly non-lethal weapons.
Experts say the incidents prove the need for studying the effect of Tasers on individuals taking prescription or illegal drugs.
The American Civil Liberties Union says at least 76 people have died in the U-S and Canada since 2001 after they were shot with Tasers.
A high percentage were using stimulants, and at least six coroner´s reports found Tasers were a contributing factor.
Officials at Arizona-based Taser International say the weapon has been vindicated.
But two September studies say more research and training are needed.