please be aware that the following is presented for informational purposes, and should not be construed as authoritative or complete. if you intend to use a substance recreationally, you should research it extensively and not rely on a single source, be that this document, a forum post, Wikipedia, Erowid, PsychonautWiki, a trained physician, or even your own medical training, if applicable.
if in any doubt, do not consume substances that could prove fatal or cause permanent damage to your body. opioids such as codeine are among those substances.
codeine is a narcotic painkiller also used in cough syrups. it is one of the only pharmaceutical opioids that can be legally accessed without a prescription in certain countries.
codeine is administered orally. there is no benefit to using any other route of administration; in particular, shooting codeine will not produce the rush that heroin users are used to.
codeine is a pro-drug for morphine. morphine is a powerful painkiller but unsuited to use outside a clinical setting, as it must be administered intravenously to avoid first-pass metabolism. when taken orally, typically only a third of the morphine survives first-pass metabolism intact. codeine is used because its metabolites include morphine - in essence, codeine is a way of smuggling morphine past the liver.
codeine is an extremely unpredictable drug compared to typical opioids. while it is well-tolerated by a small majority, its metabolic pathway depends on the enzyme CYP2D6, which is absent in some and overexpressed in others. this means that for some people, codeine will have no effect at all, and for others, even low doses can be extremely dangerous.
very few doctors or harm reduction sources are aware of the issue of ultrarapid codeine metabolism, which, while found only among a minority, is still relatively common.
note that codeine is not the only drug, or even the only opioid, that depends on CYP2D6 metabolism.
only take codeine at a known safe dose you have taken within the past two days unless there is another person present to administer naloxone and call 911. note that in the case of a codeine overdose you will likely not be able to administer naloxone yourself even if conscious, as codeine, unlike most opioids, can cause muscle paralysis and uncontrollable tremors. make sure to establish whether or not you are an ultrarapid CYP2D6 metabolizer. if you are, and you have not taken codeine before or it has been more than a week since your last dose, do not take a dose larger than 60mg.
codeine interacts synergistically with other central nervous system depressants and drug mixing can easily cause a fatal overdose. under no circumstances should codeine be mixed with depressants such as alcohol, promethazine, GHB, benzodiazepines, barbiturates, or other opioids.
as with most opioids, stimulants such as cocaine can mask a codeine overdose, leading to fatal respiratory depression when the stimulant wears off.
a number of substances can affect how codeine is metabolized. grapefruit juice may potentiate the conversion of codeine to morphine according to some sources, although it's not clear if this is based in science or anecdote.
many codeine products contain acetaminophen (paracetamol), a non-narcotic analgesic with an extremely low therapeutic index that can cause fatal liver damage in doses greater than 4g (4000mg) over 24-hour period. make sure the number of pills you're ingesting does not contain a dangerous dose of acetaminophen, and be sure to factor in other products you are using (such as migraine pills like Excedrin, or grocery-store cold remedies, which often contain acetaminophen),
the commonly quoted figure for a lethal dose of codeine is 800mg. this figure is only correct for the small majority that metabolize codeine inefficiently. doses as low as 100mg may lead to an overdose in those who metabolize codeine rapidly, and this is only an estimate - you are not automatically safe because you're taking less than that. everybody's metabolism is different and it's important to approach dose increases with extreme caution. the difference between a pleasant high and a fatal overdose may be as little as a single pill. in this way, even legitimately-prescribed pharmaceutical codeine can be as dangerous as street heroin.
if you do not live in a country where codeine can be legally possessed without a prescription, consider taking the following precautions: