http://abcnews.go.com/Nightline/story?id=4173792&page=1

Something else has to be involved for Heath Ledger to have died from the antianxietants he was taking, namely Diazepam (Valium) and Alprazolam (Xanax).  Both are fast acting and with relative fast metabolism, although Alprazolam I think has a longer half-life.  On their own, it would take a boat load to cause death, the lethal dose is astronomical. The only time these meds can cause death is when other medications/drugs are involved concurrently.  So, the question in my mind is what was the other non-prescription medication (because there were non-Rx things found at his bedside).   Or, did he have sleep apnea?  Apparently people with Sleep Apnea can die from a benzo O.D. due to higher susceptibility to respiratory cessation. 

Anyways, what a tragedy. He had everything to live for.   The first film I ever saw with him in it was Two Hands (starring Brian Brown), where he was a teen thug.  Very well done.  

Rest in Peace, Heath.