Bechler had been taking Xenadrine, an over-the-counter drug containing ephedra, which has been linked to heatstroke and heart trouble, Dr. Joshua Perper said.
Bechler died Monday, less than 24 hours after a spring training workout sent his temperature to 108.
Ephedra has been banned by the NCAA, the NFL and the International Olympic Committee, but not by major league baseball. Perper urged baseball to ban the drug.
The preliminary autopsy findings indicated Bechler died from complications of heatstroke that caused multi-organ failure.
Final results won't be available until toxicology tests are completed in two-to-three weeks, Perper said.
Among the other factors contributing to Bechler's death, Perper said, were that he had high blood pressure and liver abnormalities.
The 6-foot-2, 239-pound Bechler was an overweight athlete pushing himself in warm, humid weather much different from the climate in his hometown of Medford, Ore. He also hadn't eaten much solid food in the two days before he fell ill.
The workout Sunday left Bechler pale and dizzy. When his condition deteriorated, he was carried from the clubhouse to an ambulance on a stretcher. He spent the night in intensive care and died Monday morning at Northridge Medical Center.
Bechler was a third-round draft pick by the Orioles in 1998. He made his major league debut last September, going 0-0 with a 13.50 ERA in three relief appearances. He was expected to begin this season with the club's new Triple-A affiliate in Ottawa.
Bechler spent most of last year at Triple-A Rochester, going 6-11 with a 4.09 ERA in 24 starts.