可待因（Codeine）
Overview
Codeine is an opioid analgesic and antitussive medication derived from the opium poppy. It is frequently prescribed for the relief of mild to moderately severe pain and is also widely recognized for its effectiveness in suppressing dry, non-productive coughs. It is often formulated in combination with paracetamol or NSAIDs to provide enhanced pain relief through synergistic effects.
Mechanism of Action
Codeine works primarily within the central nervous system. As an analgesic, it binds to opioid receptors in the brain, altering the way the body perceives and responds to pain signals. As an antitussive, it acts directly on the cough center in the medulla oblongata of the brain, elevating the threshold required to trigger a cough reflex, thereby reducing the urge to cough.
Dosage and Administration
For pain management in adults, the usual dosage is 15 mg to 60 mg every 4 to 6 hours as needed. For cough suppression, lower doses are typically used. It is imperative to use the lowest effective dose for the shortest possible duration. Codeine is metabolized in the liver into morphine; however, genetic variations mean some individuals metabolize it too quickly or too slowly, affecting its efficacy and safety.
Side Effects
The most frequently reported side effects include drowsiness, dizziness, nausea, vomiting, and significant constipation. Because it is a central nervous system depressant, codeine can cause respiratory depression, particularly in high doses or in sensitive individuals. Patients should avoid alcohol and operating heavy machinery while taking this medication.
Contraindications and Risks
Codeine is contraindicated in children under 12 years of age, and in children under 18 following tonsillectomy or adenoidectomy, due to severe risks of life-threatening respiratory depression. It carries a high risk of tolerance, physical dependence, and addiction with prolonged use.
Pregnancy and Lactation
Codeine should be used during pregnancy only if clearly needed, as prolonged use can lead to neonatal withdrawal. It is strongly contraindicated during breastfeeding because the drug and its active metabolite, morphine, are excreted in breast milk, potentially causing fatal respiratory depression in the nursing infant.
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