You will not overdose on 90mg of Codeine. In fact, you will probably need to take the entire bottle simply to experience the true effects of Opiates. In case you are unaware, Codeine (3-Methylmorphine) is metabolized in vivo to Morphine at a rate of approximately 10%. This varies from person to person based upon their unique physiology, specifically, their levels of enzyme activity. At any rate, you should not be in any danger consuming the entire amount and I would actually reccomend it if you want to have a good time.
There are individuals who are hypersensitive to Opiates, but it's a very, very uncommon occurence. It's stated in various medical literature that the minimum lethal dosage of Morphine is around 200mg, but sensitive individuals can have a serious reaction to as little as 60mg. My point is that you're not even in the ballpark of a lethal dose. Drink the contents and enjoy.