Analgesics and Pain Management

• Nociceptive pain pathways: Tissue injury activates peripheral nociceptors (C-fibers and A-delta fibers) that release substance P and glutamate. Signals travel via the spinothalamic tract to the thalamus and cortex. NSAIDs and acetaminophen work peripherally; opioids work at multiple levels including central dorsal horn.

• Inflammatory cascade and prostaglandin synthesis: Tissue damage triggers phospholipase A2 release of arachidonic acid. Cyclooxygenase (COX-1 and COX-2) converts arachidonic acid to prostaglandins (especially PGE2 and PGI2), which sensitize nociceptors and amplify pain signals. NSAIDs inhibit COX enzymes; acetaminophen works through unclear mechanisms (possibly central COX inhibition and TRPV1 channel activation).

• Opioid receptor signaling: Endogenous opioids (enkephalins, dynorphins, beta-endorphins) and exogenous opioids bind mu (μ), kappa (κ), and delta (δ) receptors on neurons. Mu receptors mediate analgesia, euphoria, and respiratory depression. Receptor activation opens potassium channels (hyperpolarization) and closes calcium channels (reduced neurotransmitter release), decreasing pain signal transmission in the dorsal horn and throughout the CNS.

• Neuropathic pain mechanisms: Nerve injury causes spontaneous ectopic firing, central sensitization, loss of inhibitory GABA signaling, and glial cell activation releasing pro-inflammatory cytokines. This explains why NSAIDs and opioids are less effective; gabapentinoids (GABA analogs) and SNRIs are preferred.

• Gate control theory: Substantia gelatinosa in the dorsal horn integrates pain signals; large myelinated A-beta fibers (touch, pressure) can "close the gate" on small fiber pain signals, explaining why rubbing an injury helps and why TENS units have limited evidence.

• Acute nociceptive pain (tissue injury, surgery, trauma): Sharp, well-localized, proportional to injury severity. Associated with sympathetic hyperactivity (tachycardia, hypertension, diaphoresis, pupil dilation). Responds reliably to NSAIDs and opioids. Examples: postoperative pain, fractures, acute inflammation.

• Chronic nociceptive pain (arthritis, cancer pain, low back pain): Dull, persistent, may lose sympathetic features. Associated with depression, anxiety, sleep disturbance, functional disability. May develop tolerance to opioids requiring dose escalation.

• Neuropathic pain (diabetic neuropathy, postherpetic neuralgia, phantom limb pain, chemotherapy-induced peripheral neuropathy): Burning, shooting, tingling, numbness. Often describes as "electric," "pins and needles," or "burning." Poorly responsive to NSAIDs and conventional opioids. Associated with allodynia (pain with non-painful stimulus) and hyperalgesia (exaggerated pain response).

• Visceral pain (biliary colic, renal colic, intestinal obstruction): Deep, aching, poorly localized, often referred to skin. Associated with autonomic symptoms (nausea, vomiting). May require stronger opioids; antispasmodics help with colicky component.

• Pain in vulnerable populations: Elderly patients may present atypically with delirium or functional decline rather than reported pain. Opioid-tolerant patients require higher doses. Patients with substance use disorder need careful assessment to distinguish legitimate pain from drug-seeking behavior.

• Pain scales and severity assessment: Numeric Rating Scale (0-10), Visual Analog Scale, or categorical scales (mild/moderate/severe). In nonverbal patients (sedated, severe dementia, infants), use behavioral scales (FLACC, PAINAD). Assess pain character, location, radiation, duration, exacerbating/alleviating factors, and functional impact.

• Pain type classification: Determine if pain is nociceptive (responds to NSAIDs/opioids) or neuropathic (use gabapentinoids/SNRIs). Neuropathic pain screening tools include DN4 (Douleur Neuropathique 4) and LANSS (Leeds Assessment of Neuropathic Symptoms and Signs). Mixed pain (e.g., cancer pain with neuropathic component) requires combination therapy.

• Underutilized diagnostic testing: Lab studies (CBC, CMP, inflammatory markers) and imaging (X-ray, MRI, CT) identify underlying causes requiring specific treatment. However, imaging often shows abnormalities unrelated to pain (e.g., asymptomatic disc herniation in 30% of people without back pain), so clinical correlation is essential.

• Assess pain etiology urgently: Red flag symptoms require imaging: sudden severe headache (subarachnoid hemorrhage), focal neurologic deficits (stroke), fever with meningismus, acute severe back pain with lower extremity weakness (cauda equina), chest pain with hemodynamic instability (MI/PE), severe abdominal pain with peritoneal signs (acute abdomen).

• Assess opioid risk before prescribing: Screen for history of substance use disorder, aberrant drug-seeking behavior, depression, anxiety, and social support. Urine drug screening and prescription drug monitoring programs (PDMP) identify polypharmacy and doctor shopping in chronic pain patients.

First-line agents by pain severity

• Mild pain (ladder step 1): Acetaminophen (maximum 4 g/day; 3 g/day in elderly/liver disease) or NSAIDs. NSAIDs are more effective than acetaminophen for inflammatory conditions. Common NSAIDs: ibuprofen (400-800 mg q6-8h), naproxen (250-500 mg q12h), indomethacin (for headache/biliary colic). Avoid in renal disease, GI bleeding history, cardiovascular disease.

• Moderate pain (ladder step 2): Add weak opioids to acetaminophen or NSAIDs. Tramadol (50-100 mg q4-6h; max 400 mg/day) combines weak mu-receptor agonism with SNRI activity; avoid in seizure disorder and serotonin syndrome risk. Codeine (30-60 mg q4-6h) has limited use due to poor efficacy and variable metabolism (depends on CYP2D6 phenotype). Consider topical agents (lidocaine patch, capsaicin cream) for localized pain.

• Severe pain (ladder step 3): Use strong opioids alone or with adjuvants. Morphine is the gold standard (immediate-release 5-15 mg q3-4h IV/SC; 10-30 mg q4h oral; extended-release 15-30 mg q12h). Hydromorphone (2-4 mg q4-6h) is 5-7 times more potent than morphine. Fentanyl (patch 12-100 mcg/72h) is 100 times morphine's potency; use only in opioid-tolerant patients. Oxycodone (5-15 mg q4-6h) is 1.5 times morphine's potency. Methadone (5-10 mg q8-12h) has long half-life (15-60 hours), variable absorption, and QT-prolongation risk; requires specialist prescri

NSAIDs (mechanism-driven toxicity)

• GI ulceration/bleeding: loss of COX-1–derived PGE2 removes mucus, bicarbonate, and mucosal blood flow. Risk rises with age, prior ulcer, anticoagulants, and steroids; the American College of Gastroenterology recommends PPI co-therapy (or a COX-2–selective agent) in high-risk patients.
• Acute kidney injury: prostaglandin loss removes afferent arteriolar vasodilation, so GFR falls in prostaglandin-dependent states (heart failure, cirrhosis, hypovolemia). The "triple whammy" of NSAID + ACEI/ARB + diuretic is a classic AKI stem. Also causes hyperkalemia, sodium retention with worsened hypertension, allergic interstitial nephritis, and analgesic nephropathy with papillary necrosis.
• Cardiovascular thrombosis: selective COX-2 inhibition suppresses endothelial PGI2 while platelet COX-1/TXA2 persists; all non-aspirin NSAIDs carry an FDA boxed warning. Celecoxib is a sulfonamide — avoid with sulfa hypersensitivity.
• Other: aspirin-exacerbated respiratory disease (asthma, polyps, aspirin sensitivity), Reye syndrome in febrile children with viral illness, and — per FDA — avoidance after 20 weeks' gestation (oligohydramnios, premature ductus arteriosus closure).

Opioids

• Respiratory depression: mu-mediated blunting of medullary CO2 responsiveness — the cause of death. Reversal is naloxone (0.4 mg IV/IM, titrated; 4 mg intranasal in the community). Its duration is shorter than most opioids, so re-dosing or infusion and observation are mandatory.
• Constipation persists without tolerance; treat prophylactically with a stimulant laxative.
• Agent-specific: meperidine → normeperidine accumulation with seizures (AGS Beers criteria: avoid in older adults); methadone → QT prolongation/torsades, requiring ECG monitoring; tramadol → seizures and serotonin syndrome; morphine metabolites accumulate in renal failure; codeine is contraindicated in children after tonsillectomy and in breastfeeding mothers (CYP2D6 ultrarapid metabolizers).

• Acetaminophen overdose: the toxin is NAPQI, not the parent drug. Best next step after a potentially toxic single ingestion is a 4-hour serum level on the Rumack–Matthew nomogram; give N-acetylcysteine. Distractor to avoid: withholding NAC because the patient presents late or the level is pending — treat empirically when timing is unknown or ingestion was staggered.
• Opioid overdose triad: miosis, respiratory depression, depressed consciousness. Naloxone is diagnostic and therapeutic, but its short half-life means recurrence — never discharge immediately after a single dose. Pinpoint pupils are absent with meperidine and can be confounded by co-ingestants.
• Tolerance develops to everything except constipation and miosis. A stem describing a stable cancer patient on chronic opioids with new obstipation wants a bowel regimen, not dose reduction.
• The "triple whammy": NSAID added to an ACE inhibitor/ARB plus diuretic in an older or volume-depleted patient → prerenal AKI from loss of afferent arteriolar prostaglandins. Stop the NSAID first.
• Neuropathic pain does not respond well to NSAIDs or plain opioids. For painful diabetic neuropathy and postherpetic neuralgia, expect gabapentinoids, SNRIs (duloxetine), or TCAs; topical lidocaine for focal postherpetic pain. Carbamazepine remains the answer for trigeminal neuralgia.
• Agent-specific traps: meperidine in renal impairment or the elderly → normeperidine seizures (AGS Beers criteria says avoid); tramadol → seizures and serotonin syndrome, especially with SSRIs; methadone → QT prolongation and a long, unpredictable half-life causing delayed respiratory depression; codeine is a prodrug requiring CYP2D6 — useless in poor metabolizers and dangerous in ultrarapid metabolizers.
• Buprenorphine is a partial mu agonist with a ceiling on respiratory depression and high receptor affinity — it precipitates withdrawal if given too soon after a full agonist.
• Prescribing safety (CDC 2022 opioid guideline): check the PDMP, avoid routine concurrent benzodiazepines, use non-opioid and non-pharmacologic therapy first for chronic pain, and offer take-home naloxone to higher-risk patients. Aspirin in a febrile child is Reye syndrome until proven otherwise.