Killed his heart? Depends on the substance. It's more likely he killed his brain, which then killed his heart, before it finally kills him, but you could argue he was already dead at stage 1 though.

If it's heroin it will be, CNS depression (brain) -> respiratory depression (body) -> heart stopping -> no blood to brain

So his lungs would stop breathing, and he would die.

im pretty high i just have to get my point across. but this is just one example how he could have died, it could have been hundreds of different drugs, but that will take really long, I already wasted time on this. it also might have not been an OD after all which would make my research really a waste. and then we got the question of ''accidental OD, and suicidal OD'' or he might have been injected by someone else and that could make it murder (<0.1% chance). So it can be Accidental OD, suicidal OD, non OD-related accident, murder, health. I cannot complete my search for the answer