The frighteningly ripped body of the 38 year old Guerrero would suggest that he may have been on steroids at the time of his death. There are still some people in the medical profession who say that steroids have no clear long-term impact on adult bodies and that there is no definitive medical study that says otherwise. I think that what we do know from the deaths of innumerable 30-40 year old professional wrestlers (Rick Rude, Curt Henning, Brian Pillman, etc., now possibly Eddie Guerrero) and from the case of baseball player Ken Caminiti is that the combo of steroids+hard drugs is likely to lead to heart problems and sudden death, even after you are off the drugs.
The anecdotal evidence is that steroids basically create a bipolar state that welcomes the addition of extra drugs into the system to make the body feel better.
One wonders when the WWE will be hauled before Congress to ask about the WWE's steroid-testing policy. Last time the WWE (then WWF) started strict testing (during Vince McMahon's steroid trafficking trial) wrestlers dropped 25-30 pounds almost overnight.