A painless method to commit suicide, which I do not recommend putting into action since suicide kills you (frfr). A high potent opioid, a benzodiazepine with strong muscle-relaxant properties together with a high dose of an alpha-2 adrenergic agonist. Also not to mention a first generation antihistamine like Doxylamine to prevent vomitting, and it potentiates the nervous system depression a bit.
Combine all four substances and you won't even notice it, I thought about it myself and this is a method which even in case an ambulance is called can't be reversed just with Narcan. So extremely unlikely to get resuscitated. But all of that is very easy to say, killing yourself and all that shit. I am only sharing this because I find it very soothing knowing there's a way out, no pain, no panic.
edit: I forgot to mention an easier method which is helium. Not painful but it does not give you any positive psychoactive effects like with opioids, so you just keep feeling like shit while dying. I believe whenever I kill myself, I at least deserve some euphoric and rewarding effects before all goes black.