I'm pretty much on board with Verona and Snavel's and others' similar theories as to how poor Heath likely medicated himself to death. It's pretty understandable and very reasonable and actually quite normal behavior for many people. His family has said - and we don't have the reports of how much of each was actually in Heath's system - that the amounts of each pill were very little.
I can't tell you how many people I know who take one over the counter med then a few minutes later feeling like it's not working - try another without letting the other med get out of their system first. My sister-in-law routinely mixes alcohol and sedatives to relax on airplane flights when all the warnings on the meds tell you and tell you not to do something like that.
I can easily see Heath, being a young strong man thinking the same, it's prescribed, or it's OTC so it's OK. He's frustrated at 4am with no sleep, so he takes another pill, later, gets up, feels his allergies coming on, so he takes an antihistimine, later, decides to take a nap, pops another pill, later still, maybe, gets so groggy he can't recall when he took what or how much and takes another...
Rumor has it Marilyn Monroe OD'd this way, she was too loopy to remember the pills she had already taken.
This could have easily happened to Heath.
However, as others have said, I'm not sure who would prescribe such strong painkillers as a sleep aid, when there are so many actual medicines for insomnia that work. Had Heath been injured previously? Maybe those were just leftovers from an earlier medical treatment and he recalled that they would knock you out, so he tried them. No report has mentioned how old all these prescriptions were.
I assume we'll get more info on this as time goes on, maybe?