From Wiki:
Alzado is probably most remembered today for being one of the first major U.S. sports figures to admit using steroids. In the last years of his life, as he battled against the brain tumor that eventually caused his death at the age of 43, Alzado asserted that his steroid abuse directly led to his fatal illness, but his physician stated it could not possibly be true. According to some reports, Alzado was using natural growth hormone, harvested from human corpses, as opposed to synthetic growth hormones.
Paul DeMayo - heroine overdose.
John Grimek - old age (88)?
Mike Mentzer - heart complications.
Ray Mentzer - complications from Berger's disease.
Andreas Münzer - died of complications due to the enormous number and combinations of weight training supplements he used while competing. After months of stomach pains, he was admitted to a hospital on the morning of March 12th. By 7 pm, doctors had decided to operate to stop bleeding in his stomach, but shortly afterwards his liver and kidneys failed. Münzer refused a blood transfusion, and died on the morning of March 13th at the age of 30.
Sonny Schmidt - cancer.
Mohammed Benaziza - diuretic overdose (Lasix) ?
Interesting that every Mr. Olympia is still alive...