Because codeine is a prodrug you can’t redose, well you can but the affects are very very minimal.

I might be wrong with exactly how it works so correct if I’m wrong someone but basically your body produces an amount CYP2D6 and when you take codeine it’s metabolized by this enzyme to produce morphine and some other active metabolites. Once it’s processed and used up it’s stores of this enzyme the remaining codeine will just pass through and have no analgesic effect.

Now when I comes to redosing your body needs to have regenerated these stores to be able to process the codeine into morphine and this takes many hours. For me after my initial dose I need to wait a good 12-16 hours before getting the same feeling again.

Some people are good at producing this enzyme and some lack it, hence why for some codeine does absolutely nothing no matter how much they take.

My wording of some things might be abit off but that’s a simple Version of how it works