Codeine
Codeine is an opiate and prodrug of morphine mainly used to treat pain, coughing, and diarrhea. It is commonly used as a recreational drug. It is found naturally in the sap of the opium poppy, Papaver somniferum. It is typically used to treat mild to moderate degrees of pain. Greater benefit may occur when combined with paracetamol as codeine/paracetamol or a nonsteroidal anti-inflammatory drug such as aspirin or ibuprofen. Evidence does not support its use for acute cough suppression in children. In Europe, it is not recommended as a cough medicine for those under 12 years of age. It is generally taken orally. It usually starts working after half an hour, with maximum effect at two hours. Its effects last for about four to six hours. Codeine exhibits abuse potential similar to other opioid medications, including a risk of addiction and overdose.
Common side effects include nausea, vomiting, constipation, itchiness, lightheadedness, and drowsiness. Serious side effects may include breathing difficulties and addiction. Whether it can be used safely during pregnancy is unclear. Care should be used during breastfeeding, as it may result in opiate toxicity in the baby. Its use as of 2016 is not recommended for children. Codeine works by being broken down by the liver into morphine; how quickly this occurs depends on a person's genetics.
Codeine was discovered in 1832 by Pierre Jean Robiquet. In 2013, about 361,000 kg of codeine were produced while 249,000 kg were used, which made it the most commonly taken opiate. It is on the World Health Organization's List of Essential Medicines. Codeine occurs naturally and makes up about 2% of opium.
Medical uses
Pain
Codeine is used to treat mild to moderate pain. It is commonly used to treat post-surgical dental pain.Weak evidence indicates that it is useful in cancer pain, but it may have increased adverse effects, especially constipation, compared to other opioids. The American Academy of Pediatrics does not recommend its use in children due to side effects. The Food and Drug Administration lists age under 12 years old as a contraindication to use.
Cough
Codeine is used to relieve coughing. Evidence does not support its use for acute cough suppression in children. In Europe, it is not recommended as a cough medicine for those under 12 years of age. Some tentative evidence shows it can reduce a chronic cough in adults.Diarrhea
It is used to treat diarrhea and diarrhea-predominant irritable bowel syndrome, although loperamide, diphenoxylate, paregoric, or even laudanum are more frequently used to treat severe diarrhea.Formulations
Codeine is marketed as both a single-ingredient drug and in combination preparations with paracetamol ; with aspirin ; or with ibuprofen. These combinations provide greater pain relief than either agent alone.Codeine is also commonly marketed in products containing codeine with other pain killers or muscle relaxers, as well as codeine mixed with phenacetin, naproxen, indomethacin, diclofenac, and others, as well as more complex mixtures, including such mixtures as aspirin + paracetamol + codeine ± caffeine ± antihistamines and other agents, such as those mentioned above.
Codeine-only products can be obtained with a prescription as a time-release tablet. Codeine is also marketed in cough syrups with zero to a half-dozen other active ingredients, and a linctus for all of the uses for which codeine is indicated.
Injectable codeine is available for subcutaneous or intramuscular injection only; intravenous injection is contraindicated, as this can result in nonimmune mast-cell degranulation and resulting anaphylactoid reaction. Codeine suppositories are also marketed in some countries.
Side effects
Common adverse effects associated with the use of codeine include drowsiness and constipation. Less common are itching, nausea, vomiting, dry mouth, miosis, orthostatic hypotension, urinary retention, euphoria, and dysphoria. Rare adverse effects include anaphylaxis, seizure, acute pancreatitis, and respiratory depression. As with all opiates, long-term effects can vary, but can include diminished libido, apathy, and memory loss. Some people may have allergic reactions to codeine, such as the swelling of the skin and rashes.As with other opioids, a potentially serious adverse drug reaction is respiratory depression. This depression is dose-related and is a mechanism for the potentially fatal consequences of overdose. As codeine is metabolized to morphine, morphine can be passed through breast milk in potentially lethal amounts, fatally depressing the respiration of a breastfed baby.
In August 2012, the United States Food and Drug Administration issued a warning about deaths in pediatric patients less than 6 years old after ingesting "normal" doses of paracetamol with codeine after tonsillectomy; this warning was upgraded to a black box warning in February 2013.
Some patients are very effective converters of codeine to its active form, morphine, resulting in lethal blood levels. The FDA is presently recommending very cautious use of codeine in young tonsillectomy patients; the drug should be used in the lowest amount that can control the pain, "as needed" and not "around the clock", and immediate medical attention is needed if the user responds negatively.
Withdrawal and dependence
As with other opiates, chronic use of codeine can cause physical dependence which can lead to severe withdrawal symptoms if a person suddenly stops the medication. Withdrawal symptoms include drug craving, runny nose, yawning, sweating, insomnia, weakness, stomach cramps, nausea, vomiting, diarrhea, muscle spasms, chills, irritability, and pain. These side effects also occur in acetaminophen/aspirin combinations, though to a lesser extent. To minimize withdrawal symptoms, long-term users should gradually reduce their codeine medication under the supervision of a healthcare professional.Also, no evidence indicates that CYP2D6 inhibition is useful in treating codeine dependence, though the metabolism of codeine to morphine does have an effect on the abuse potential of codeine. However, CYP2D6 has been implicated in the toxicity and death of neonates when codeine is administered to lactating mothers, particularly those with increased enzyme activity.
In 2019 Ireland was said to be on the verge of a codeine addiction epidemic, according to a paper in the Irish Medical Journal. Under Irish law, codeine can be bought over the counter under the supervision of a pharmacist, but there is no mechanism to detect patients travelling to different pharmacies to purchase codeine.
Pharmacology
Pharmacodynamics
| Compound | Route | Dose |
| Codeine | 200 mg | |
| Hydrocodone | 20–30 mg | |
| Hydromorphone | 7.5 mg | |
| Hydromorphone | 1.5 mg | |
| Morphine | 30 mg | |
| Morphine | 10 mg | |
| Oxycodone | 20 mg | |
| Oxycodone | 10 mg | |
| Oxymorphone | 10 mg | |
| Oxymorphone | 1 mg |
Codeine is a nonsynthetic opioid. It is a selective agonist of the μ-opioid receptor. Codeine itself has relatively weak affinity for the MOR. Instead of acting directly on the MOR, codeine functions as a prodrug of its major active metabolites morphine and codeine-6-glucuronide, which are far more potent MOR agonists in comparison.
Codeine has been found as an endogenous compound, along with morphine, in the brains of nonhuman primates with depolarized neurons, indicating that codeine may function as a neurotransmitter or neuromodulator in the central nervous system. Like morphine, codeine causes TLR4 signaling which causes allodynia and hyperalgesia. It does not need to be converted to morphine to increase pain sensitivity.
Mechanism of action
Codeine is an opiate and an agonist of the μ-opioid receptor. It acts on the central nervous system to have an analgesic effect. It is metabolised in the liver to produce morphine which is ten times more potent against the MOR. Opioid receptors are G protein-coupled receptors that positively and negatively regulate synaptic transmission through downstream signalling. Binding of codeine or morphine to the μ-opioid receptor results in hyperpolarization of the neuron leading to the inhibition of the release of nociceptive neurotransmitters, causing an analgesic effect and increased pain tolerance due to reduced neuronal excitability.Pharmacokinetics
The conversion of codeine to morphine occurs in the liver and is catalyzed by the cytochrome P450 enzyme CYP2D6. CYP3A4 produces norcodeine, and UGT2B7 conjugates codeine, norcodeine, and morphine to the corresponding 3- and 6-glucuronides. Srinivasan, Wielbo, and Tebbett speculate that codeine-6-glucuronide is responsible for a large percentage of the analgesia of codeine, and thus these patients should experience some analgesia.CYP2D6 converts codeine into morphine, which then undergoes glucuronidation. Life-threatening intoxication, including respiratory depression requiring intubation, can develop over a matter of days in patients who have multiple functional alleles of CYP2D6, resulting in ultrarapid metabolism of opioids such as codeine into morphine.
Studies on codeine's analgesic effect are consistent with the idea that metabolism by CYP2D6 to morphine is important, but some studies show no major differences between those who are poor metabolizers and extensive metabolizers. Evidence supporting the hypothesis that ultrarapid metabolizers may get greater analgesia from codeine due to increased morphine formation is limited to case reports.
Due to the increased metabolism of codeine to morphine, ultrarapid metabolizers are at increased risk of adverse drug effects related to morphine toxicity. Guidelines released by the Clinical Pharmacogenomics Implementation Consortium advise against administering codeine to ultrarapid metabolizers, where this genetic information is available. The CPIC also suggests that codeine use be avoided in poor metabolizers, due to its lack of efficacy in this group.
Codeine and its salts are readily absorbed from the gastrointestinal tract, and ingestion of codeine phosphate produces peak plasma concentrations in about one hour. Plasma half life is between 3 and 4 hours, and oral/intramuscular analgesic potency ratio is approximately equal to 1:1.5. The most common conversion ratio, given on equianalgesia charts used in the United States, Canada, the UK, Republic of Ireland, the European Union, Russia and elsewhere as 130 mg IM equals 200 mg PO—both of which are equivalent to 10 mg of morphine sulphate IV and 60 mg of morphine sulphate PO. The salt:freebase ratio of the salts of both drugs in use are roughly equivalent, and do not generally make a clinical difference.
Codeine is metabolised by O- and N-demethylation in the liver to morphine and norcodeine. Hydrocodone is also a metabolite of codeine in humans. Codeine and its metabolites are mostly removed from the body by the kidneys, primarily as conjugates with glucuronic acid.
The active metabolites of codeine, notably morphine, exert their effects by binding to and activating the μ-opioid receptor. People that can metabolize codeine to an extensive degree could result in a 30 mg dose yielding 4 mg of morphine.