PH3.4 | PH3.4 | Opioid Analgesics and Safe Use Instructions — SDL Guide — Summary & Reflection

KEY TAKEAWAYS

Opioid analgesics act via Gi-coupled μ, κ, δ receptors — inhibiting adenylyl cyclase, opening K⁺ channels, and closing pre-synaptic Ca²⁺ channels. Full μ-agonists (morphine, codeine, fentanyl, pethidine) have no ceiling on respiratory depression. Buprenorphine is a partial μ-agonist — ceiling on respiratory depression, high receptor affinity. Naloxone is a pure antagonist (IV — overdose reversal; short t½ — re-narcotisation risk). Overdose triad: coma + miosis + respiratory depression. Critical safe-use rules: pethidine contraindicated in renal failure (norpethidine → seizures) and with MAOIs (excitatory crisis); tramadol risks serotonin syndrome with SSRIs/MAOIs and seizures in epilepsy. Always co-prescribe a laxative with chronic opioids (tolerance does not develop to constipation).

REFLECT

A 68-year-old man with prostate cancer metastatic to bone is admitted with severe, uncontrolled bone pain (NRS 9/10). He is on escitalopram (SSRI) for depression and has an eGFR of 28 mL/min/1.73m² (Stage 4 CKD). A junior colleague suggests prescribing pethidine for acute pain control, followed by tramadol for ongoing analgesia. Walk through your pharmacological reasoning: why are both suggestions potentially dangerous for this specific patient, and what would you prescribe instead, with what monitoring plan?