1. Clinical Overview
Aspirin (Acetylsalicylic Acid) 150mg is a non-steroidal anti-inflammatory drug (NSAID) with potent antiplatelet, analgesic, antipyretic, and anti-inflammatory properties. In the Indian context, the 150mg strength is primarily used for its antiplatelet effect in the secondary prevention of cardiovascular and cerebrovascular events. It irreversibly inhibits platelet cyclooxygenase-1 (COX-1), preventing the formation of thromboxane A2, a key promoter of platelet aggregation and vasoconstriction.
| Onset | Duration | Bioavailability |
|---|---|---|
| Analgesic/Antipyretic: 30-60 minutes. Antiplatelet: Within 60 minutes of ingestion, with significant inhibition occurring within 1 hour. | Analgesic/Antipyretic: 4-6 hours. Antiplatelet: 7-10 days (lifespan of affected platelets). | Approximately 50-70% for oral doses due to pre-systemic hydrolysis in the gut wall and liver. |
2. Mechanism of Action
Aspirin's primary mechanism at the 150mg dose is the irreversible acetylation of a serine residue (Ser529 in human COX-1) at the active site of the enzyme cyclooxygenase-1 (COX-1), predominantly in platelets. This inhibits the conversion of arachidonic acid to prostaglandin H2, thereby blocking the synthesis of thromboxane A2 (TXA2), a potent inducer of platelet aggregation and vasoconstriction. Platelets, being anucleate, cannot synthesize new COX-1, so the effect lasts for the platelet's lifespan (7-10 days). It also has a dose-dependent inhibitory effect on COX-2, involved in inflammation and pain, but this is more significant at higher doses (>325mg).
3. Indications & Uses
- Secondary prevention of myocardial infarction (MI)
- Secondary prevention of ischemic stroke and transient ischemic attack (TIA)
- Secondary prevention in patients with stable or unstable angina pectoris
- Prevention of thrombosis after coronary artery bypass grafting (CABG) or percutaneous coronary intervention (PCI) with stenting (usually in dual antiplatelet therapy with a P2Y12 inhibitor like clopidogrel)
4. Dosage & Administration
Adult Dosage: For secondary cardiovascular prevention: 75-150mg once daily. 150mg once daily is a common and effective dose in Indian practice. Should be taken with food and a full glass of water to minimize gastric upset.
Administration: Take exactly as prescribed, usually once daily. Can be taken in the morning or evening. Take with or immediately after food to reduce gastric irritation. Do not crush or chew enteric-coated tablets. Swallow whole with a full glass of water. Do not lie down for at least 30 minutes after taking. If a dose is missed, take it as soon as remembered. If it is near the time for the next dose, skip the missed dose. Do not double the dose.
5. Side Effects
Common side effects may include:
- Dyspepsia, heartburn, epigastric discomfort
- Nausea
- Increased bleeding tendency (e.g., easy bruising, prolonged bleeding from cuts)
- Tinnitus (at higher doses, rare at 150mg)
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Warfarin / Acenocoumarol | Increased risk of major bleeding due to synergistic antiplatelet/anticoagulant effect. | Major |
| Clopidogrel / Prasugrel / Ticagrelor | Synergistic antiplatelet effect used therapeutically in ACS/PCI. Also increases bleeding risk. | Major (Therapeutic but requires monitoring) |
| Ibuprofen / Naproxen (NSAIDs) | Competitive, reversible inhibition of platelet COX-1 can antagonize aspirin's irreversible antiplatelet effect if taken concomitantly. Ibuprofen should be taken at least 2 hours after aspirin. | Major |
| Corticosteroids (e.g., Prednisolone) | Increased risk of GI ulceration and bleeding. | Moderate |
| Selective Serotonin Reuptake Inhibitors (e.g., Sertraline, Escitalopram) | Increased risk of upper GI bleeding due to impaired platelet function. | Moderate |
| Methotrexate | Decreased renal clearance of methotrexate, leading to increased toxicity (myelosuppression). | Major |
| ACE Inhibitors (e.g., Ramipril, Enalapril) | Aspirin may attenuate the vasodilatory and cardioprotective benefits of ACE inhibitors. | Moderate |
| Antacids | Alkaline environment can increase urinary excretion of salicylate and may slightly reduce aspirin absorption. | Minor |
| Valproic Acid | Aspirin displaces valproic acid from protein binding sites, increasing free valproate levels and risk of toxicity. | Moderate |
| Herbal (Ginkgo Biloba, Garlic) | Increased antiplatelet effect and bleeding risk. | Moderate |
7. Patient Counselling
- DO take it exactly as prescribed by your doctor, usually once daily.
- DO take it with food and a full glass of water to protect your stomach.
- DO inform all your doctors and dentists that you are on aspirin before any procedure.
- DO report any signs of unusual bleeding (black/tarry stools, blood in urine/vomit, excessive bruising) immediately.
- DON'T stop taking aspirin suddenly without consulting your cardiologist/physician, as it may increase your risk of heart attack or stroke.
- DON'T take other painkillers like ibuprofen, diclofenac, or naproxen without asking your doctor, as they can interfere with aspirin's heart-protective effect.
- DON'T chew or crush enteric-coated tablets.
- DON'T consume alcohol while on this medication.
8. Toxicology & Storage
Overdose: Mild: Nausea, vomiting, tinnitus, vertigo, hyperventilation. Moderate: Severe metabolic acidosis, respiratory alkalosis, hyperthermia, dehydration, electrolyte imbalances. Severe: CNS effects (lethargy, seizures, coma), pulmonary edema, renal failure, cardiovascular collapse, severe hemorrhage. Chronic salicylism can occur with repeated high therapeutic doses.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep the container tightly closed. Keep out of reach and sight of children. Do not use after the expiry date printed on the pack.