Though the world saw him as an untouchable Rock genius, behind the scene, Elvis Presley was dealing with a lot of personal troubles. One of those troubles was an addiction to drugs. His first introduction to drugs came when Presley was in the military. He was introduced to amphetamines.
He did not look at them in a negative way, as street drugs are looked at because they were medicine. Later on his life, a doctor introduced him to opiates, to bring him down from the amphetamines. This was a deadly mix. Tying the drugs in with an unhealthy lifestyle and obesity was a perfect case for a heart attack.