codeine depends on an individuals biology and the production of enzymw CYP2D6
a low dose for one person could floor another person depending on their production of that enzyme
some will nod of 30mg others will take 400mg and fell nothing, except maybe itchy
"Codeine is not itself centrally active, and must first be converted via first-pass metabolism into morphine by the cytochrome P450 enzyme CYP2D6 (as such, it is a prodrug for morphine). Codeine is also metabolized into the inactive norcodeine via the CYP3A4 enzyme system. Both resultant forms are conjugated by UGT2B7 into their corresponding 3-glucuronide.[citation needed]
Some percentage of the population produces less CYP2D6 enzymes and so experience a significant reduction of effects from codeine in comparison to that of the average person. However, others produce CYP2D6 enzymes in higher quantities which can result in hypersensitivity to the drug.....This also indicates a lacking capability in the metabolism of codeine into morphine when a user consumes antihistamines such as diphenhydramine prior to the ingestion of codeine. [7]
There is an upper limit to the amount of codeine which can be converted by enzymatic metabolism into morphine throughout an individual session. This limit is commonly referred to as the "ceiling dose", which is commonly believed to be around 400mg. Consuming higher doses will lead to greater side effects such as itchiness and nausea, but will not increase euphoria. "
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