How easily a medication can be used for suicide is one of the factors used to decide which ones are over the counter or not, generally by design the over the counter medications need an absurdly higher than intended dose to be taken before giving a high chance of death and not just of surviving with damage.
This lethal dose is so much higher than the intended dose that it would be hard to take it in the timeframe required, like you'd need to swallow such a huge amount of pills, or drink a huge amount if it's some kind of syrup, that it would be quite hard to do fast enough and without vomiting.
You can find some exceptions for x country at some point in time, but these are usually just oversights that quickly become prescriptions when someone realizes, nowadays in most countries there's probably no medication left for it without prescription, as preventing it is taken very seriously nowadays.
Of course I don't know what country you live in and how competent they are at preventing this, so it might be possible that there is a medication in your country that is decent enough to be a considerable method. I don't know why you'd choose otc medications as the method though, as you can see it's quite well prevented nowadays so finding an exception in your country would take some good research, and it would probably still be mediocre compared to other methods