Codeine

Codeine is an opiate, meaning it is one of the alkaloids obtained naturally from the opium poppy, Papaver somniferum, where it makes up a small percentage of opium; it can also be produced from morphine ^[1]. Chemically and pharmacologically it belongs to the opioid analgesics, the same family as morphine, but it is much weaker in its own right, and most of its effect depends on being changed into morphine inside the body ^[1][3]. It is commonly formulated on its own or combined with non-opioid painkillers such as paracetamol, aspirin or ibuprofen ^[1].

Codeine was first isolated in 1832 in France by the chemist Pierre-Jean Robiquet, making it one of the earliest purified plant alkaloids to be used in medicine ^[1]. Over the following two centuries it became one of the most frequently taken opioids in the world, valued for being effective by mouth and comparatively mild ^[1].

In medicine codeine is used chiefly for mild to moderate pain, often in combination products, and as a cough suppressant, although the evidence does not support its use for coughs in children ^[1]. It is also used to reduce the frequency of stools in diarrhoea, including diarrhoea-predominant irritable bowel syndrome, because opioids slow the movement of the gut ^[1]. Its usefulness is tempered by concerns about safety and by the availability of alternatives ^[4].

A defining and clinically important feature of codeine is that people vary enormously in how they process it, because the enzyme that converts it to morphine, CYP2D6, differs from person to person ^[2][3]. Individuals known as ultrarapid metabolisers turn codeine into morphine unusually quickly and can develop dangerously high morphine levels from ordinary doses, a mechanism linked to serious and even fatal breathing problems, including in young children after tonsil surgery and in breastfed infants whose mothers took codeine ^[2][4]. At the opposite extreme, poor metabolisers make little morphine and gain little pain relief ^[2][3]. Because of these risks, professional guidelines recommend that CYP2D6 genotype be taken into account when prescribing codeine ^[3], and regulators have contraindicated it in young children and issued strong warnings about its use in children and during breastfeeding ^[2][4].

As an opioid, codeine carries the class's characteristic side effects, including constipation, nausea, drowsiness and, at high doses, dangerous suppression of breathing, and regular use can lead to tolerance, physical dependence and addiction, with withdrawal symptoms on stopping ^[1]. It is a controlled substance in most countries, with the level of control depending on the dose and formulation; some low-strength combination products are sold from pharmacies in certain countries while higher-strength forms require a prescription ^[1]. Codeine is also misused recreationally, notably in cough-syrup mixtures known by names such as lean, and it appears on the World Health Organization Model List of Essential Medicines ^[1].

The signature danger with codeine is that you cannot predict how strong it will be. Because the active morphine is made by CYP2D6, ultrarapid metabolizers can turn a routine dose into a toxic amount of morphine and slip into life-threatening respiratory depression; this has killed children given codeine after tonsil and adenoid surgery, and it is why the FDA now bars codeine in kids under 12 and after that surgery in anyone under 18.

Drugs that block CYP2D6, like the antidepressants fluoxetine, paroxetine, and bupropion, can blunt codeine by choking off morphine formation. Like every opioid it brings constipation, drowsiness, nausea, itching, and dose-related slowing of breathing, and regular use builds tolerance, physical dependence, and a real withdrawal syndrome; it is misusable and can be a stepping stone to stronger opioids.

Mixing it with alcohol, benzodiazepines, gabapentinoids, or other sedatives stacks the respiratory depression and is a common way people die. The cough-syrup misuse trend known as "lean" or "purple drank" combines codeine syrup with promethazine and soda, layering an opioid on top of a sedating antihistamine, and it has caused fatal overdoses. Legally it is a controlled substance in the US whose exact schedule depends on the formulation; pure or high-dose codeine sits in Schedule II, acetaminophen or aspirin combinations in Schedule III, and low-dose cough preparations in Schedule V, with several countries restricting it far more tightly.

Codeine is a prodrug that requires conversion by CYP2D6 to morphine to produce analgesia. Selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors that inhibit CYP2D6 (particularly fluoxetine and paroxetine) substantially reduce codeine's effectiveness by lowering morphine formation. This is a pharmacokinetic interaction; the antidepressant reduces codeine's conversion to its active metabolite. Studies document that concomitant use leads to inadequate pain control, with one retrospective analysis finding that 0.44% of patients in a real-world French cohort received this problematic combination despite the documented risk ^[5]. Oxycodone, which is metabolized by the same enzyme, shows a similar interaction pattern; observational data found that initiating oxycodone while taking the CYP2D6-inhibiting SSRIs paroxetine or fluoxetine carried a 23% increased risk of opioid overdose compared to other SSRIs ^[6].

Codeine's interactions with most other drugs are not formally documented. Although the enzyme inhibition concept is clear, specific clinical outcomes from combining codeine with non-psychiatric CYP2D6 inhibitors (such as certain antihistamines or antiarrhythmics) have not been published.

Checking a whole stack? Run it through interactions + stacks.