Pain Relievers — Medication Guide, Uses & Safety — Uses, Dosage & Side Effects | MyMedicPlus

Pain relievers, or analgesics, are medications used to reduce or eliminate pain ranging from mild headaches and dental pain to severe post-surgical, cancer-related, and neuropathic (nerve) pain. They represent one of the most commonly used and important categories of medication across all healthcare settings. Analgesics are classified into several distinct categories based on their mechanism of action and clinical application: non-opioid analgesics (paracetamol/acetaminophen), non-steroidal anti-inflammatory drugs (NSAIDs — ibuprofen, naproxen, diclofenac, indomethacin, celecoxib), opioid analgesics (codeine, tramadol, morphine, oxycodone, hydrocodone, fentanyl, buprenorphine, tapentadol), and adjuvant analgesics (anticonvulsants such as gabapentin and pregabalin, tricyclic antidepressants such as amitriptyline, SNRIs such as duloxetine — all used for neuropathic and chronic pain). The WHO analgesic ladder — originally developed for cancer pain but broadly applied — recommends a stepwise approach: starting with non-opioid analgesics for mild pain, progressing to weak opioids (codeine, tramadol) for moderate pain, and strong opioids (morphine, oxycodone, fentanyl) for severe pain, with adjuvant agents added at each step for specific pain types. Multimodal pain management combining different analgesic classes at lower doses of each is generally preferred over monotherapy at high doses.

Analgesics treat pain across a broad spectrum of conditions and pain types. Nociceptive pain (from tissue damage) includes: headache, migraine (paracetamol, NSAIDs, triptans, aspirin), dental pain, post-operative pain, musculoskeletal pain (back pain, osteoarthritis, sports injuries, fibromyalgia), menstrual cramps (NSAIDs are first-line — COX inhibition reduces uterine prostaglandins), ear pain, and renal or biliary colic. Neuropathic pain (from nerve damage or dysfunction) — such as diabetic peripheral neuropathy, postherpetic neuralgia, trigeminal neuralgia, spinal cord injury pain, and chemotherapy-induced peripheral neuropathy — is best treated with adjuvant analgesics: gabapentin and pregabalin (first-line), duloxetine, amitriptyline, or topical lidocaine patches. Cancer pain typically requires a stepwise opioid-based approach tailored by palliative care specialists. Chronic pain syndromes (fibromyalgia, low back pain, complex regional pain syndrome) are best managed with multimodal approaches combining medication, physical therapy, and psychological interventions. Opioids are prescribed by pain specialists, oncologists, palliative care physicians, and GPs for severe acute and cancer pain, and require careful risk-benefit assessment for chronic non-cancer pain.

Each analgesic class operates through a distinct neurochemical mechanism. Paracetamol (acetaminophen) acts primarily in the central nervous system, inhibiting prostaglandin synthesis in the brain and spinal cord (via COX inhibition and potentially through the endocannabinoid system and serotonergic pathways), raising the pain threshold and reducing fever, with minimal peripheral anti-inflammatory action. NSAIDs (ibuprofen, naproxen, diclofenac, celecoxib) non-selectively or selectively inhibit COX-1 and COX-2 cyclooxygenase enzymes both centrally and peripherally, blocking prostaglandin production at the site of tissue injury, reducing inflammation, edema, and peripheral sensitization of pain receptors (nociceptors). The reduction in prostaglandin E2 and I2 at both peripheral and spinal cord levels reduces the transmission and amplification of pain signals. Opioid analgesics (morphine, oxycodone, fentanyl, codeine) bind to G-protein-coupled mu, kappa, and delta opioid receptors in the brain (periaqueductal gray matter, thalamus, limbic system), spinal cord dorsal horn, and peripheral sensory neurons. Receptor activation inhibits adenylyl cyclase, reduces cAMP, closes voltage-gated calcium channels, opens inwardly rectifying potassium channels — collectively hyperpolarizing neurons and reducing the release of pain-transmitting neurotransmitters (substance P, glutamate), blocking nociceptive signal transmission. Gabapentin and pregabalin bind to the alpha-2-delta subunit of voltage-gated calcium channels in spinal cord neurons, reducing the release of excitatory neurotransmitters (substance P, glutamate) from overactivated pain neurons — most effective for neuropathic, central sensitization, and post-operative pain. Tricyclic antidepressants (amitriptyline) inhibit serotonin and norepinephrine reuptake, enhancing descending inhibitory pain pathways from the brain to the spinal cord.

Dosing varies significantly by analgesic class, route, and pain severity. Paracetamol: 500–1000 mg every 4–6 hours (max 4000 mg/day adults; 2000 mg/day in liver disease or regular alcohol use). Ibuprofen: 200–400 mg every 4–6 hours with food (max 1200 mg/day OTC; up to 3200 mg/day prescription). Naproxen: 250–500 mg twice daily (max 1250 mg/day). Codeine: 30–60 mg every 4–6 hours (max 240 mg/day); often combined with paracetamol (co-codamol). Tramadol: 50–100 mg every 4–6 hours (max 400 mg/day); extended-release 100–400 mg once daily. Morphine (immediate release): 5–30 mg every 4 hours; (extended-release): 10–200 mg every 12 hours. Gabapentin: 300 mg at night initially, titrated to 300–1200 mg three times daily (max 3600 mg/day). Pregabalin: 25–150 mg twice daily (max 300 mg twice daily). Amitriptyline for neuropathic pain: 10–25 mg at night, titrated to 25–75 mg. Topical diclofenac gel: apply 2–4 g to affected joint 3–4 times daily. Opioid doses require individualized medical titration starting at the lowest effective dose. Never self-adjust prescription analgesic doses.

Each analgesic class has a distinct and important side effect profile. Paracetamol: extremely safe at recommended doses; the critical risk is hepatotoxicity (liver damage) from overdose — including accidental overdose from hidden paracetamol in combination cold, flu, and pain products — or from regular excessive use combined with alcohol. Even modestly exceeding the daily maximum for several days can cause severe liver injury. Rare: serious skin reactions (Stevens-Johnson syndrome). NSAIDs: gastrointestinal toxicity (stomach irritation, ulcers, GI bleeding — always take with food; consider PPI gastroprotection in high-risk patients); kidney impairment (particularly in dehydrated patients, elderly, or those with pre-existing renal disease); cardiovascular risk (elevated heart attack and stroke risk with long-term regular use — FDA black-box warning); fluid retention worsening heart failure and hypertension; platelet dysfunction increasing bleeding time. Avoid in pregnancy third trimester, severe CKD, active peptic ulcer. Opioids: constipation (extremely common, occurs in 40–70% and does not develop tolerance — always co-prescribe a stimulant laxative such as senna); nausea and vomiting (common initially); sedation and cognitive impairment; respiratory depression (potentially fatal, especially in opioid-naive patients, with dose escalation, or with concomitant CNS depressants); itching (pruritis); urinary retention; physical dependence (expected with prolonged use — gradual dose tapering required when stopping); addiction (opioid use disorder — behavioral, psychological, and physiological); and opioid-induced hyperalgesia (paradoxical increased pain sensitivity with long-term high-dose opioid use). Adjuvant analgesics: gabapentin/pregabalin — dizziness and somnolence (very common — approximately 20–30%), peripheral edema, blurred vision, and weight gain; both are associated with dependence and misuse risk and require gradual tapering when stopping. Amitriptyline — anticholinergic effects (dry mouth, constipation, urinary retention, confusion in elderly), QT prolongation, and cardiotoxicity in overdose.

Opioids carry FDA black-box warnings for addiction, abuse, misuse, and potentially fatal respiratory depression, particularly when combined with benzodiazepines (diazepam, temazepam, clonazepam), alcohol, other CNS depressants, or gabapentinoids — these combinations dramatically increase the risk of respiratory depression and death. Never combine opioids with benzodiazepines without explicit medical supervision and only when alternatives have been exhausted. Opioid use disorder (OUD) is a recognized medical condition — seek treatment and support if you are concerned about dependence. Naloxone (Narcan) — an opioid receptor antagonist — can rapidly reverse opioid overdose and should be available in households where opioids are prescribed for high-risk patients. Paracetamol overdose is a medical emergency — seek emergency care immediately even if symptoms are absent for the first 12–24 hours, as liver failure may not manifest until 24–72 hours after ingestion, by which point it may be too late for effective treatment with N-acetylcysteine. Key drug interactions: NSAIDs with warfarin (significantly increased bleeding risk — avoid or monitor INR closely); NSAIDs with ACE inhibitors/ARBs and diuretics (triple whammy kidney failure risk); tramadol with SSRIs, SNRIs, or MAOIs (serotonin syndrome risk — potentially fatal); opioids with MAOIs (potentially fatal — avoid); gabapentin/pregabalin with opioids and benzodiazepines (increased respiratory depression risk — FDA warning). Long-term NSAID use requires monitoring kidney function, blood pressure, and consideration of gastric protection. For patients with chronic pain, a comprehensive multimodal approach incorporating physical therapy, psychological pain management, and appropriate non-pharmacological strategies is strongly encouraged to minimize medication burden and adverse effects.