Codeine is a synthetic drug derived from morphine. It was first discovered in 1832 by Pierre Robiquet, a French chemist. Small amounts of codeine are actually found in the opium poppy from which morphine is extracted, although modern formulations make codeine from pharmaceutical-grade morphine.
How does it work?
The drug works by interacting with opiate receptors in the brain, reducing the sensation of pain in the same way as morphine and the related (illicit) drug heroin. This is why drugs related to morphine and codeine are known as opiates.
Codeine is a much weaker narcotic analgesic than morphine – approximately one-tenth the potency.
It doesn’t work for everyone
About 8% of the population is unable to metabolise codeine to its active metabolite, morphine, leading to a poor response to codeine. This is not in itself dangerous (these people just have to change analgesics), but there are also a small number of people (about 5%) who metabolise codeine to morphine at a much larger extent.
These people are at an increased risk of toxicity caused by the increased amounts of morphine produced.
In Australia, codeine is available over the counter by itself or in combination with a range of other drugs in low-dose formulations (8mg, 15mg) such as with the anti-inflammatory drug ibuprofen. Stronger tablets (30mg) are only available by prescription.
Its main use is to relieve pain and to treat minor aches, including headaches, but only for a short term. Doctors can prescribe a higher-dose form (forte formulations containing 30mg codeine) for more significant aches and pains.
Low-dose forms are also used to treat coughs (antitussive), often available as syrup or linctus. Codeine can also help reduce the effects of nausea and diarrhoea without causing significant side effects.
Codeine is the most widely and commonly used prescription opiate in the world. It is relatively cheap when obtained over-the-counter, often for less than A$10 for low-dose packs.
Higher costs are associated with combination tablets and larger pack sizes. Prescribed codeine phosphate 30mg is available in a pack size of 20 tablets costing a maximum of A$25.23 under the PBS.
It is taken orally with doses ranging from 15-60mg but may range up to 240mg daily. Once consumed this drug will act for approximately three to six hours.
Codeine is available in combination preparations such as Nurofen Plus, which also contains the anti-inflammatory drug ibuprofen; and in cold and flu preparations with paracetamol and decongestants. Codeine is also found in combination with paracetamol and doxylamine (for example Mersyndol). This is recommended for the treatment of tension headache, migraine, and it may be useful in controlling fever.
The drug should not be used for prolonged periods, particularly without supervision and control by a medical practitioner. The main risk is the development of dependence (addiction) and tolerance that can lead to use of higher doses, particularly in patients whose pain is not well managed.
Codeine may not be the best treatment for a given condition; hence medical supervision is always warranted when longer-term use is sought. Risks are also associated with products that combine codeine with other drugs. Prolonged use of high-dose codeine/ibuprofen combinations has been linked with gastrointestinal disorders and renal failure. Combinations with paracetamol can also lead to liver damage.
Codeine may cause red, itchy skin rashes, difficulty breathing, faintness, constipation, hayfever, and swelling of the face or throat.
Serious side effects include unusual sleepiness, confusion, and difficult and noisy breathing. The drug can suppress the cough reflex and create breathing difficulties, particularly when asleep.
Adverse drug combinations
Alcohol (other than low doses), other analgesics, benzodiazepines (Valium, Temaze, Serepax), other sedatives and sleeping tablets, and some antidepressants interact with codeine and should be avoided since they may enhance the sedative actions.
Use of codeine is not advised in people who have had recent biliary tract surgery (unblocking of the bile ducts), have suffered a recent head injury or conditions that raise the pressure within the head, or are suffering from diarrhoea caused by poisoning or antibiotics.