"Where is the information about codeine's ceiling dose coming from? It would be nice to at least see a reference if claims like this, along with calculations supporting them, are going to be made. Just read the wiki article and you'll see that the ceiling dose effect contradicts other facts mentioned in the article. At the start of the pharmacology section, there is mention that 70% of codeine is metabolized to codeine-6-glucuronide (C6G). Codeine is metabolized to C6G by uridine diphosphate glucuronosyl transferase (UGT) 2B7. Only about 5% of codeine is metabolized by CYP 2D6 (Armstrong and Cozza, 2003). The article cited states that C6G is responsible for codeine analgesia, not morphine via 2D6. The pharmacology section is directly contradicting itself. The claims I have ever seen about the codeine "ceiling effect" have not been backed up by evidence: they seem to lie on the assumption that codeine analgesia is due to CYP2D6 producing morphine, which is being refuted. I have never seen any good evidence to suggest that codeine has a ceiling effect and some who use codeine recreationally will tell you that their experience contradicts it. I suggest removing this nonsense from the article.
Armstrong SC and Cozza KL. (2003) Pharmacokinetic drug interactions of morphine, codeine, and their derivatives: theory and clinical reality, Part II. Psychosomatics. 44: 515 - 520."
Not sure about DHC to be honest, but the codeine ceiling dose not mean effects wont be increased by upping the dosage, however the dose response range will drop.