If I were to down an entire bottle of pills and go to sleep, would I be able to end it just like that, with little pain or suffering? Would my death be guaranteed assuming no one checks in on me for 12+ hours?
It really depends on what you take.
I overdosed a few times.
With one of them I was super sick but obviously didn't die. I stayed up all night vomiting and then some and couldn't keep anything down. I could barely get up or down a flight of stairs for a week following it because I was so physically weak. I remember just laying on my side and vomiting, not even into a bucket or anything. I got some rash on my fave from the acid coming down the side of my mouth. I was too weak to even vomit in a bucket. It was miserable.
Another time I was apparently hallucinating and vomiting a lot all though I don't remember it very well. I was told I was delirious.
Experiences could be different for different drugs.
I would recommend an anti-emetic.