Aspirin for pain relief

Generic Name

Acetylsalicylic acid

Brand Names

Aspirin

Aspro

Disprin

Ecotrin

Bufferin

Bayer Aspirin

Anadin

ASA

Various global generics and branded preparations exist, including enteric-coated and effervescent forms

Drug Class

Nonsteroidal anti-inflammatory drug (NSAID)

Salicylate

Antiplatelet agent (low-dose use)

Analgesic, antipyretic, anti-inflammatory agent (standard/high dose use)

Mechanism of Action in Pain Relief

Aspirin exerts its analgesic and anti-inflammatory effects primarily through irreversible inhibition of the cyclooxygenase (COX) enzymes—COX-1 and COX-2

This inhibition leads to decreased synthesis of prostaglandins and thromboxane A2 from arachidonic acid

Prostaglandins are key mediators of pain, inflammation, and fever in peripheral tissues and the central nervous system

By reducing peripheral prostaglandins at the site of tissue injury or inflammation, aspirin decreases nociceptor sensitization and attenuates pain signals

It also acts centrally on hypothalamic prostaglandin synthesis to lower fever and mildly reduce central pain perception

Unlike other NSAIDs, aspirin irreversibly acetylates COX enzymes, particularly platelet COX-1, contributing to its antiplatelet effect at lower doses

Indications in Pain Management

Approved or Widely Accepted Uses for Pain Relief

Mild to moderate pain (headache, toothache, musculoskeletal pain)

Tension-type headache

Migraine (mild to moderate)

Dysmenorrhea (menstrual cramps)

Myalgia, arthralgia, joint pain

Post-operative pain

Dental pain

Sore throat, pharyngitis

Backache, lumbago

Pain from minor injuries or inflammation

Rheumatic pain (including osteoarthritis, acute rheumatoid arthritis—though less used today due to alternatives)

Not Suitable For

Severe visceral pain (e.g. renal colic)

Neuropathic pain

Chronic pain requiring strong opioid therapy

Postoperative pain with bleeding risk

Pain in patients with GI ulcers or aspirin sensitivity

Off-Label or Less Common Uses in Pain Management

Adjunct in pericarditis-associated chest pain

Adjunct in Kawasaki disease for inflammation and fever

Used in combination with opioids in compound analgesic formulations

Dosage and Administration for Pain Relief

Adults

Usual analgesic dose: 300–900 mg every 4–6 hours as needed

Maximum dose: 4 g per 24 hours

Taken with food or after meals to reduce gastrointestinal irritation

Effervescent tablets or dispersible forms may be preferred for faster onset

Avoid taking aspirin on an empty stomach

Extended-release formulations are not appropriate for pain relief

Elderly

Use with caution due to higher risk of gastrointestinal bleeding and renal impairment

Start with the lowest effective dose

Avoid prolonged use without medical supervision

Children

Aspirin is contraindicated in children under 16 years of age due to the risk of Reye's syndrome, particularly in the context of viral infections

Exception: used under specialist guidance for Kawasaki disease or juvenile idiopathic arthritis

Alternative analgesics (e.g. paracetamol, ibuprofen) preferred in pediatric pain management

Renal or Hepatic Impairment

Use with caution

Dose adjustment may be necessary

Avoid in severe renal or hepatic failure

Pregnancy

Avoid during third trimester due to risk of premature closure of the ductus arteriosus, prolonged labor, and bleeding

Occasional single-dose use in first and second trimesters may be acceptable, but not recommended without physician advice

Low-dose aspirin (75–100 mg/day) may be used under supervision for preeclampsia prevention or antiphospholipid syndrome

Lactation

Excreted in breast milk in small amounts

Occasional use acceptable but prolonged use discouraged due to theoretical risk of platelet dysfunction or Reye-like symptoms in infants

Pharmacokinetics

Absorption: rapid in stomach and upper small intestine

Peak plasma concentration: ~30–60 minutes (immediate-release); faster with effervescent or dispersible forms

Half-life: aspirin ~15–20 minutes; salicylic acid (active metabolite) ~3–12 hours depending on dose

Metabolism: hepatic (deacetylated to salicylate)

Excretion: renal, pH-dependent clearance

Onset of analgesia: within 30–60 minutes

Contraindications

Known hypersensitivity to aspirin or NSAIDs (e.g. aspirin-exacerbated respiratory disease)

History of peptic ulcer disease or GI bleeding

Hemophilia or other bleeding disorders

Severe renal or hepatic impairment

Active bleeding or high bleeding risk

Children and adolescents under 16 years with viral illness

Pregnancy in third trimester

Gout (may worsen uric acid retention)

Asthma with known aspirin sensitivity

Warnings and Precautions

May cause gastrointestinal irritation, ulceration, or bleeding—use with food and gastroprotective agents (e.g. PPI) if needed

Caution in elderly, alcohol use, or those on anticoagulants

Avoid use with other NSAIDs or high-dose corticosteroids due to increased bleeding risk

Can provoke bronchospasm in susceptible asthmatic individuals

Avoid in dehydration or renal insufficiency due to risk of nephrotoxicity

Caution in patients with hypertension—may interfere with antihypertensive medications

Monitor for tinnitus or hearing loss at high doses (early sign of salicylism)

Risk of Reye’s syndrome in children and teens—avoid in viral infections

May interfere with lab tests for liver function, uric acid, and thyroid hormones

Adverse Effects

Common

Gastrointestinal irritation

Nausea

Dyspepsia

Epigastric discomfort

Heartburn

Less Common

Gastric ulcer

Gastrointestinal bleeding

Bruising

Tinnitus (dose-related)

Prolonged bleeding time

Allergic reactions (skin rash, urticaria)

Rare but Serious

Peptic ulcer perforation or hemorrhage

Bronchospasm or anaphylaxis

Reye's syndrome (children)

Salicylism (toxicity with high doses): tinnitus, vomiting, confusion, metabolic acidosis

Acute kidney injury (especially in volume depletion)

Hematologic effects (thrombocytopenia, leukopenia)

Liver injury (rare, dose-dependent)

Pregnancy and Lactation Warnings

Avoid in third trimester

Risk of bleeding, fetal toxicity

Occasional use in early pregnancy unlikely to cause harm but not encouraged

May be excreted in breast milk—avoid prolonged use in breastfeeding mothers

Drug Interactions

Anticoagulants (warfarin, DOACs)

Increased risk of bleeding

Avoid combination unless benefit outweighs risk and patient is monitored closely

Other NSAIDs or corticosteroids

Additive GI toxicity and bleeding risk

SSRIs (e.g. fluoxetine, sertraline)

Increased GI bleeding risk

Methotrexate

Aspirin may reduce renal excretion of methotrexate, increasing toxicity risk

ACE inhibitors, ARBs, diuretics

Possible decreased antihypertensive effect

Increased risk of renal impairment with triple therapy (ACEi + diuretic + NSAID)

Uricosuric agents (probenecid, sulfinpyrazone)

Aspirin interferes with uric acid excretion—antagonizes effect

Phenytoin, valproic acid

Displacement from plasma proteins—may increase free active drug levels

Alcohol

Additive risk of gastrointestinal bleeding

Monitoring Parameters

Pain relief and duration of action

Signs of GI bleeding (e.g. black stools, hematemesis)

Renal function with long-term use

Signs of allergic reaction (especially in asthma)

Hearing and tinnitus in chronic or high-dose therapy

Counseling Points

Take after meals or with food to minimize stomach upset

Do not crush or chew enteric-coated tablets

Avoid alcohol during aspirin therapy

Do not combine with other NSAIDs unless directed

Use the lowest effective dose for the shortest duration

Store in a dry, cool place—aspirin is hydrolyzed by moisture

Report symptoms of bleeding, black stools, vomiting blood, or persistent stomach pain

Keep out of reach of children

Not suitable for children or teenagers for fever or pain

Avoid if you are pregnant unless advised by your doctor

Comparative Notes

Aspirin vs Paracetamol

Both relieve mild to moderate pain and fever

Paracetamol is preferred in patients with ulcers, asthma, or bleeding risk

Aspirin has anti-inflammatory action; paracetamol does not

Aspirin carries higher gastrointestinal and bleeding risk

Aspirin vs Ibuprofen

Ibuprofen is less irritating to the stomach and has fewer GI risks with short-term use

Both have anti-inflammatory and analgesic properties

Ibuprofen reversibly inhibits COX enzymes, unlike aspirin’s irreversible effect

Aspirin is preferred in secondary cardiovascular prevention; ibuprofen is not

Aspirin vs Naproxen

Naproxen has longer duration of action

Both may be used for musculoskeletal pain or arthritis

Naproxen preferred in inflammatory pain; aspirin less used for chronic pain today

Regulatory and Legal Status

Available over-the-counter in many countries

Listed in the WHO Model List of Essential Medicines

Available in tablets (chewable, enteric-coated, dispersible), powders, effervescent forms, suppositories

Also combined with other analgesics or caffeine in compound OTC pain relief products

Prescription-only for high doses in many jurisdictions