Pharmacologic Management of Pain
Analgesic strategy and safety
Use multimodal, mechanism-matched therapy. Acetaminophen is a first-line adjunct; keep total intake at or below 3 g/day in many adults, lower with liver disease or alcohol use. NSAIDs help inflammatory and nociceptive pain but carry gastrointestinal, renal, and cardiovascular risk; avoid in chronic kidney disease, heart failure, ulcer history, and anticoagulation. Opioids can be appropriate for severe acute pain or selected chronic cancer and noncancer pain after risk assessment; start low, titrate, and monitor function and adverse effects. Adjuvants include gabapentinoids for neuropathic pain with renal dosing, TCAs and SNRIs such as duloxetine or venlafaxine for neuropathic and centralized pain, topical lidocaine or capsaicin, and short-term muscle relaxants.
Know equianalgesia and incomplete cross-tolerance. Common oral approximations: morphine 30 mg is about oxycodone 20 mg or hydromorphone 7.5 mg; when rotating, reduce 25-50% and reassess. Meperidine is avoided for chronic pain because of normeperidine neurotoxicity. Tramadol lowers seizure threshold and can cause serotonin syndrome. Methadone prolongs QT. Buprenorphine is a partial mu agonist with a ceiling on respiratory depression but not analgesia; avoid abrupt cessation.
Traps: tolerance is not addiction; dependence is expected; opioid-induced hyperalgesia presents as worsening pain with dose escalation. Serotonin syndrome can occur with tramadol, SNRIs, and MAOIs. NSAID plus anticoagulant increases bleeding risk.
Worked example: rotate morphine 60 mg/day orally to oxycodone using a 1.5:1 ratio: 60/1.5 = 40 mg/day; reduce 25% for incomplete cross-tolerance = 30 mg/day, divided and reassessed. Rule: start low, go slow, and titrate to function.