Most over the counter stuff isn't going to kill you in a reasonable amount. As in, it'd take a fuck ton of a otc to actually kill you (like maybe multiple bottles), so you'd either vomit before you took the right amount, or if you managed to keep it in it'd be yucky horrible death (think like liver failure). Very slow and agonizing. I'm not one to critique a method, since at the end of the day it's your life and death, but in terms of overdoses, an opioid one would be a better alternative, or even like a benoz (although I'm sure those have their caveats also)
Like even trying to think of something more potent, the best I can come up with is diphenhydramine (Benadryl), and even that you'd trip spiders or something and it'd have to be really high doses and again, yucky horrible agonizing death. I might be wrong here though since my recreational drugs knowledge is limited.
There's a thread on here that explains more about this stuff and definitely more people on here who know more than just a silly seal, but with what I know I'd advise against it and try to find something else, my sorry :(