Opioid Pharmacology

Opioids are appropriate for acute severe pain — postoperative pain, trauma, and pain from acute medical conditions such as renal colic — particularly when non-opioid analgesics are insufficient. The current standard is multimodal analgesia: combining opioids with acetaminophen, nonsteroidal anti-inflammatory drugs (when not contraindicated), and regional anesthetic techniques to achieve adequate pain control at lower individual doses of each agent. This approach reduces total opioid consumption and adverse effect burden compared to opioids as the sole analgesic.

For acute pain treated in the outpatient setting, prescriptions should be limited to the expected duration of severe pain — typically three to seven days — with specific instructions for tapering and guidance on safe disposal of unused medications. Overprescribing of acute opioids is a recognized entry point for long-term opioid use and opioid use disorder.

First published in 1986 for cancer pain management and subsequently applied broadly, the World Health Organization analgesic ladder organizes pain management in three steps based on pain severity. It provides a rational framework for analgesic selection and dose escalation.

The guiding principles of the World Health Organization ladder are often summarized as: by mouth (oral route preferred when possible), by the clock (around-the-clock dosing for persistent pain rather than as-needed dosing that allows pain to return before the next dose), and by the ladder (stepwise titration based on response). For cancer pain, there is no fixed upper dose limit — the appropriate dose is the lowest that provides adequate pain control with acceptable adverse effects.

The role of opioids in chronic non-cancer pain — conditions such as chronic low back pain, fibromyalgia, and osteoarthritis — is substantially more limited and contested than their role in cancer pain. Long-term opioid therapy for chronic non-cancer pain carries risks of tolerance, physical dependence, opioid use disorder, and opioid-induced hyperalgesia that may outweigh the analgesic benefit, particularly when non-opioid alternatives (physical therapy, cognitive behavioral therapy, topical agents, antidepressants, anticonvulsants) have not been adequately tried. Current guidelines from the United States Centers for Disease Control and Prevention emphasize non-opioid therapies as first-line treatment for chronic pain and recommend reserving opioids for patients in whom the expected benefits outweigh risks, using the lowest effective dose and regularly reassessing the ongoing need.

The most clinically important opioid consideration in renal impairment is the accumulation of active and neuroexcitatory metabolites that are renally cleared. Morphine should be used with caution or avoided in significant renal impairment because morphine-6-glucuronide accumulates and causes prolonged respiratory depression. Meperidine should be avoided entirely in renal impairment because normeperidine accumulates and causes seizures. Fentanyl and hydromorphone are generally preferred in renal impairment because their metabolites are less pharmacologically active and less likely to accumulate to toxic levels. Oxycodone requires dose reduction in renal impairment.

Because virtually all opioids are hepatically metabolized, severe hepatic impairment reduces first-pass metabolism, slows clearance, and increases bioavailability of oral formulations. Dose reductions and extended dosing intervals are required. Buprenorphine undergoes extensive hepatic metabolism and should be used cautiously in patients with severe hepatic impairment; it is relatively contraindicated in patients with acute hepatitis or decompensated liver disease because of reports of hepatotoxicity at high doses. Methadone's complex pharmacokinetics are further complicated by hepatic disease.

Elderly patients are at heightened risk for opioid adverse effects for several reasons: reduced hepatic and renal clearance prolongs opioid half-lives; decreased protein binding increases free drug concentration; reduced respiratory reserve lowers the threshold for clinically significant respiratory depression; and age-related cognitive vulnerability increases the risk of opioid-induced delirium. The general principle is to start at lower doses and titrate more slowly in elderly patients. Meperidine is explicitly listed on the Beers Criteria of potentially inappropriate medications in older adults because of its neuroexcitatory metabolite normeperidine and should be avoided in this population.

All opioids cross the placenta and can produce neonatal respiratory depression at delivery, requiring neonatal monitoring and availability of naloxone. For pregnant women with opioid use disorder, methadone and buprenorphine maintenance are both recommended as preferred treatments — abrupt opioid withdrawal in pregnancy carries risks of fetal distress, preterm labor, and miscarriage that outweigh the risks of continued opioid maintenance. The goal is a stable maternal opioid level rather than abstinence.

Infants born to mothers maintained on opioids — or who used illicit opioids during pregnancy — will be physically dependent at birth and develop neonatal opioid withdrawal syndrome within 24 to 72 hours of delivery. The syndrome manifests as irritability, high-pitched crying, poor feeding, tremors, vomiting, diarrhea, and in severe cases seizures. Treatment involves supportive care and, when necessary, pharmacological management with oral morphine or methadone, with gradual weaning. Neonatal opioid withdrawal syndrome is not a contraindication to maternal opioid use disorder treatment — it is a manageable neonatal condition, and the risks of untreated maternal opioid use disorder are substantially greater.