Codeine is an opiate, found naturally in the opium poppy, used for pain relief and for feelings of euphoria. It is also the world's most commonly-used opiate. Codeine is processed in the human liver to the more potent "Codeine-6-glucuronide" (referred to later on as C6G), and to a lesser extent morphine, and so is considered largely a prodrug; however, it's also somewhat active by itself. The level to which a person can achieve a high from codeine depends a great deal on how much codeine they can process into C6G and morphine and how quickly.
First and foremost it should be noted that the vast majority of codeine preparations come with a secondary drug in them, such as acetaminophen, ibuprofen, or another NSAID to help prevent abuse. It would be beneficial to do a Cold water extraction so you don't take too much acetaminophen/ibuprofen. Since it is possible that you may be allergic to codeine, it's best if you do not take more than 30mg to 60mg on your first dose. A codeine allergy is quite unpleasant and will result in more than just itching, as in difficulty breathing and facial swelling. For a user without any sort of opiate tolerance, a dose of 1.1mg/k to 2mg/k on an empty stomach is generally best for a recreational dose. Some users with an opiate tolerance will only feel euphoric after 2.2mg/k to 3mg/k. This translates to around 150-180mg for a low dose, 180-280mg for a medium dose, and 280-400mg for a high dose. Generally, if codeine is used along with 50mg to 100mg of promethazine the euphoric effects will be doubled. Taking more than 400mg codeine is not productive, as your body can't process any more past that point. The majority of codeine's analgesic and euphoric effects come from its metabolization into C6G by the enzyme UGT2B7, and only about 5% of the codeine ingested is metabolized into morphine by the CYP2D6 enzyme.
 Route of administration
Oral use is the preferred route of administration, as it's generally agreed that for the best possible effects, codeine must be processed in the liver so that it can become C6G and morphine. Snorting is therefore not advised, and IV use can be deadly (see Allergies).
There is some contention about how well rectal administration of codeine works. Some claim it is far superior to oral administration, however.
 Typical Effects
- Mild euphoria
- Simultaneous stimulation and sedation
- Constricted pupils
- Mild difficulty focusing eyes
- Dizziness at higher doses or when mixed with other drugs
- Nausea at higher doses or when mixed
- Shortness of breath, again, at higher doses or when mixed
- Shortened temper and increased agitation at relatively small things
- Weak alcohol-esque hangover
Codeine is the opiate most likely to cause an allergic reaction. Even in those not allergic, it can cause a histamine reaction, leading to the opiate itch. This is intensified when codeine is injected, and can be potentially lethal; therefore, never inject codeine.
A study found that roughly 8% of those of European descent can't process codeine into C6G and morphine, meaning that it has very little use for them. Dextromethorphan was introduced in 1958 to replace codeine phosphate as the active ingredient in most cough suppressants.