Aspirin; Uses, Dosage, Side Effects, Interactions

Aspirin is the prototypical analgesic used in the treatment of mild to moderate pain. It has anti-inflammatory and antipyretic properties and acts as an inhibitor of cyclooxygenase which results in the inhibition of the biosynthesis of prostaglandins. Aspirin also inhibits platelet aggregation and is used in the prevention of arterial and venous thrombosis.

Aspirin is a Nonsteroidal Anti-inflammatory Drug and Platelet Aggregation Inhibitor. The mechanism of action of aspirin is as a Cyclooxygenase Inhibitor. The physiologic effect of aspirin is by means of Decreased Prostaglandin Production and Decreased Platelet Aggregation. The chemical classification of aspirin is Nonsteroidal Anti-inflammatory Compounds.

Aspirin is an orally administered non-steroidal anti-inflammatory agent. Acetylsalicylic acid binds to and acetylates serine residues in cyclooxygenases, resulting in decreased synthesis of prostaglandin, platelet aggregation, and inflammation. This agent exhibits analgesic, antipyretic, and anticoagulant properties.

Aspirin is a common medicine that has a number of uses, from relieving pain to reducing the risk of serious problems such as heart attacks and strokes.

Mechanism of Action of Aspirin

The analgesic, antipyretic, and anti-inflammatory effects of acetylsalicylic acid are due to actions by both the acetyl and the salicylate portions of the intact molecule as well as by the active salicylate metabolite. Acetylsalicylic acid directly and irreversibly inhibits the activity of both types of cyclooxygenase (COX-1 and COX-2) to decrease the formation of precursors of prostaglandins and thromboxanes from arachidonic acid. This makes acetylsalicylic acid different from other NSAIDs (such as diclofenac and ibuprofen) which are reversible inhibitors. Salicylate may competitively inhibit prostaglandin formation. Acetylsalicylic acid’s antirheumatic (nonsteroidal anti-inflammatory) actions are a result of its analgesic and anti-inflammatory mechanisms; the therapeutic effects are not due to pituitary-adrenal stimulation. The platelet aggregation-inhibiting effect of acetylsalicylic acid specifically involves the compound’s ability to act as an acetyl donor to cyclooxygenase; the nonacetylated salicylates have no clinically significant effect on platelet aggregation. Irreversible acetylation renders cyclooxygenase inactive, thereby preventing the formation of the aggregating agent thromboxane A2 in platelets. Since platelets lack the ability to synthesize new proteins, the effects persist for the life of the exposed platelets (7-10 days). The acetylsalicylic acid may also inhibit production of the platelet aggregation inhibitor, prostacyclin (prostaglandin I2), by blood vessel endothelial cells; however, inhibition prostacyclin production is not permanent as endothelial cells can produce more cyclooxygenase to replace the non-functional enzyme.

Aspirin acetylates prostaglandin endoperoxide synthase (prostaglandin G/H-synthase) and irreversibly inhibits its cyclooxygenase (COX) activity. The enzyme catalyzes the conversion of arachidonic acid to PGH2, the first committed step in prostanoid biosynthesis. Two isoforms of prostaglandin endoperoxide synthase exist, PGHS-1 and PGHS-2 (also referred to as COX-1 and COX-2, respectively). PGHS-1 (COX-1) is expressed constitutively in most cell types, including platelets. PGHS-2 (COX-2) is undetectable in most mammalian cells, but its expression can be induced rapidly in response to mitogenic and inflammatory stimuli. Aspirin is a relatively selective inhibitor of platelet PGHS-1 (cyclooxygenase-1, COX-1). The existence of 2 isoenzymes with different aspirin sensitivities, coupled with extremely different recovery rates of their cyclooxygenase (COX) activity following inactivation by aspirin, at least partially explains the different dosage requirements and durations of aspirin effects on platelet function versus the drug’s analgesic and anti-inflammatory effects. Human platelets and vascular endothelial cells process PGH2 to produce thromboxane A2 and prostacyclin (epoprostenol, PGI2), respectively. Thromboxane A2 induces platelet aggregation and vasoconstriction, while prostacyclin inhibits platelet aggregation and induces vasodilation. Aspirin is antithrombotic in a wide range of doses inhibiting thromboxane A2and prostacyclin.

Indications of Aspirin

At high doses – usually 300mg – aspirin can relieve pain, reduce a high temperature (fever) and reduce swelling.

Therapeutic Indications of Aspirin

  • Anti-Inflammatory Agents, Non-Steroidal; Cyclooxygenase Inhibitors; Fibrinolytic
  • Low doses of aspirin (<100 mg daily) are used widely for their cardioprotective effects.
  • Salicylates are indicated to relieve myalgia, musculoskeletal pain, and other symptoms of nonrheumatic inflammatory conditions such as athletic injuries, bursitis, capsulitis, tendinitis, and nonspecific acute tenosynovitis.
  • Salicylates are indicated for the symptomatic relief of acute and chronic rheumatoid arthritis, juvenile arthritis, osteoarthritis, and related rheumatic diseases. Aspirin is usually the first agent to be used and may be the drug of choice in patients able to tolerate prolonged therapy with high doses.
  • These agents do not affect the progressive course of rheumatoid arthritis. Concurrent treatment with a glucocorticoid or a disease-modifying antirheumatic agent may be needed, depending on the condition being treated and patient response.
  • Salicylates are also used to reduce arthritic complications associated with systemic lupus erythematosus.
  • Salicylates are indicated to reduce fever and inflammation in rheumatic fever. However, they do not prevent cardiac or other complications associated with this condition.
  • Sodium salicylate should be avoided in rheumatic fever if congestive cardiac complications are present because of its sodium content. Also, large doses of any salicylate should be avoided in rheumatic fever if severe carditis is present because of possible adverse cardiovascular effects.
  • Aspirin is indicated in the treatment of men who have had transient brain ischemia due to fibrin platelet emboli to reduce the recurrence of transient ischemic attacks and the risk of stroke and death.
  • Aspirin is also used in the treatment of women with transient brain ischemia due to fibrin platelet emboli. However, its efficacy in preventing stroke and death in female patients has not been established.
  • Aspirin is also indicated in the treatment of patients with documented, unexplained transient ischemic attacks associated with mitral valve prolapse. However, if transient ischemic attacks continue to occur after an adequate trial of aspirin therapy, aspirin should be discontinued and an oral anticoagulant administered instead.
  • Aspirin is also indicated to prevent initial or recurrent cerebrovascular embolism, transient ischemic attacks, and stroke following carotid endarterectomy. Aspirin is indicated in the treatment of patients who have had a completed thrombotic stroke, to prevent a recurrence.
  • Aspirin is indicated for its anti-inflammatory, antipyretic, and antithrombotic effects in the treatment of Kawasaki disease (Kawasaki syndrome, mucocutaneous lymph node syndrome) in children.
  • It reduces fever, relieves inflammation (e.g., lymphadenitis, mucositis, conjunctivitis, serositis), and may reduce the occurrence of cardiovascular complications. However, the combination of high-dose intravenous gamma globulin and aspirin has been shown to be more effective than aspirin alone in reducing the formation of coronary artery abnormalities.
  • Thrombotic thrombocytopenic purpura is a severe multisystemic disorder of unknown origin. The association of relapsing thrombotic thrombocytopenic purpura with pregnancy is rare but well documented and high mortality rates of mothers and fetuses have been reported so far.
  • Since the introduction of plasma therapy for treating the acute exacerbations of the disease, overall mortality rates have decreased significantly. It is now evident that the manifestations of the disease may reappear even after long disease-free intervals and as many as a thirds of the recovering patients may develop a relapse.
  • Presented are two thrombotic thrombocytopenic purpura patients with relapsing thrombotic thrombocytopenic purpura complicating their pregnancies. Prophylactic treatment with aspirin and dipyridamole during their last three successful pregnancies prevented or minimized the severity of thrombotic thrombocytopenic purpura relapses.

Contra-Indications of Aspirin

Most people can take aspirin safely. But you should get advice from a pharmacist or doctor before taking it if you:

Dosage of Aspirin

Strengths:  75 mg, 150 mg, 300 mg 500 mg; 600 mg , 800 mg

Myocardial Infarction


  • Initial dose: 160 to 162.5 mg orally once as soon as myocardial infarction is suspected
  • Maintenance dose: 160 to 162.5 mg orally once a day for 30 days post-infarction



  • 300 to 650 mg orally every 4 to 6 hours as needed
  • Maximum dose: 4 g in 24 hours


  • 300 to 600 mg rectally every 4 hours

Ischemic Stroke

  • Immediate-release: 50 to 325 mg orally once a day
  • Extended-release (ER): 162.5 mg orally once a day

Ischemic Stroke – Prophylaxis

  • Immediate-release: 50 to 325 mg orally once a day
  • Extended-release (ER): 162.5 mg orally once a day

Angina Pectoris Prophylaxis

  • Immediate-release (IR): 75 mg to 325 mg orally once a day
  • Extended-release (ER): 162 mg orally once a day

Angina Pectoris

  • Immediate-release (IR): 75 mg to 325 mg orally once a day
  • Extended-release (ER): 162 mg orally once a day

Side Effects of Aspirin

The most common

More common

Less common

Drug Interactions of Aspirin

Aspirin may interact with following drugs, supplements & may decrease the efficacy of the drug

Pregnancy & Lactation of Aspirin

FDA Pregnancy Category N – Not Assigned


US FDA pregnancy category Not Assigned: The US FDA has amended the pregnancy labeling rule for prescription drug products to require labeling that includes a summary of risk, a discussion of the data supporting that summary, and relevant information to help healthcare providers make prescribing decisions and counsel women about the use of drugs during pregnancy. Pregnancy categories A, B, C, D, and X are being phased out.


Low quantities of salicylates and of their metabolites are excreted into the breast milk. Since adverse effects for the infant have not been reported up to now, short-term use of the recommended dose does not require suspending lactation. In cases of long-term use and/or administration of higher doses, breastfeeding should be discontinued.