A dose of 200mg oral morphine can be enough to be fatal in some opioid naive people, however differences in the way people absorb/process the substance has massive bearing on how it actually affects you.
Opioids alone are generally not recommended due to this, and to the fact that you can be easily rescued with narcan by EMTs.
There is a also worrying chance of lasting effects that can make your life worse should you attempt and fail - google opioid overdose and you'll find a wealth of information about this.
There are major differences between different substances too; if you're using something weak like cocodamol, you're likely to suffer greatly due to the paracetamol content rather than the opioid content; oral morphine has low bioavailability which means that a lot of it is kind of filtered out by your digestive system.
As ClairyFairy (cute name!) mentioned, the hard stuff like H and F are sought out for ctb purposes, especially injected, as this bypasses the filtration of digestion.
But still not wholly reliable as methods go, and scary to think of what lasting effects you might be left with if you live through the attempt.
So tempting though as the thought of just passing out in a euphoric haze sounds amazing! I take morphine daily and have been so taken by this idea that I've taken doses far above what is considered a fatal dose and felt barely anything, or just thrown up the following day after a nice long sleep lol