Eight years after Heath Ledger's death, his father is opening about the last time he spoke with his late son.In an interview with Australia's News.com , Kim Ledger says that the night before Heath's passing from an accidental prescription drug overdose, the actor's sister warned him about mixing medications.“It was a one-off thing,” the elder Ledger said. “That’s what killed us, because he was warned by his sister the night before: 'You shouldn’t mix what you’re taking for pneumonia with your Ambien.'"Ledger said Heath had a very "cavalier boy's answer" to the whole thing that night. “He said, ‘Katie, Katie, I’ll be fine,'" the father explained, but that the combination "just put his whole system to sleep, I guess." He added that his son always had a problem with sleeping, even as a child, and his hectic work schedule didn't help things.“Heath mixed a couple of drugs together with sleeping tablets and he’s gone forever," Ledger told News.com. "That’s something we [his relatives] just have to deal with.” He hopes that by telling his son's story he can help others who are struggling with addiction all over the world.In the same article, addiction medicine specialist Christian Rowan, MP, said that a quarter of all Australians admit to using opioids every month, with 450 dying every year due to painkiller misuse. The Centers for Disease Control and Prevention (CDC) reported that in 2014, more than 14,000 Americans died from overdoses involving prescription opioids.“We encourage people to start verbalizing their problems with close friends or families or other relatives,” Ledger said. “Losing a child is something that never leaves you. You can live with the pain, but it never leaves you."