1. Meperidine is highly addictive, and overdose can cause generalized seizures. It is not reversible by naloxone. When taken with monoamine oxidase inhibitors (MAOIs), meperidine can cause serotonergic syndrome, characterized by hyperthermia, excitation, delusions, and seizures (Melzack and Wall 2003).
2. Codeine, dihydrocodeine, and diamorphine are prodrugs of morphine. They are converted to active forms by CYP2D6 enzymes in the liver (Thorn et al. 2009). Patients lacking this enzyme cannot metabolize medications containing codeine, resulting in treatment failure. Conversely, patients who are ultra-metabolizers can experience toxicity.
3. Morphine causes histamine release which can lead to pruritus. Furthermore, morphine is glucuronidated in the liver to an active metabolite, morphine-6-glucuronide, which is then excreted via the kidney (Warfield and Bajwa 2004). Therefore, morphine should not be used in renal failure patients.
4. Rapid infusion of large doses of fentanyl can cause increased muscle tone of the thorax leading to chest wall rigidity and the development of rigid chest syndrome (Ballantyne 2009). Also the Food and Drug Administration (FDA) has recently issued warnings due to deaths in patients using fentanyl patches and there is no one specific dose that causes this potentially lethal syndrome.
5. Methadone is very long acting, with a 23 h half-life. It has been associated with torsades de pointes, therefore a baseline electrocardiogram is now recommended prior to initiating treatment (Krantz et al. 2009).
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