Codeine
Uses
Codeine is an opiate used to treat pain, coughing, and diarrhea. It is typically used to treat mild to moderate degrees of pain. Greater benefit may occur when combined with paracetamol (acetaminophen) or a nonsteroidal anti-inflammatory drug (NSAID) such as aspirin or ibuprofen. It is generally taken by mouth.
Side Effects
Common side effects include vomiting, constipation, itchiness, lightheadedness, and drowsiness. Serious side effects may include breathing difficulties and addiction. Whether its use in pregnancy is safe is unclear. Care should be used during breastfeeding, as it may result in opiate toxicity in the baby.
Pharmacodynamics
Codeine is an opioid. It is a selective agonist of the μ-opioid receptor (MOR). 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 metabolite morphine, which is a far more potent MOR agonist in comparison
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.
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.
Codeine is metabolised by O- and N-demethylation in the liver to morphine and norcodeine. Codeine and its metabolites are excreted almost entirely by the kidney, mainly as conjugates with glucuronic acid.