The Discovery and Evolution of Aspirin in Medicine

Aspirin is unique as the only NSAID with antiplatelet effects due to irreversible COX-1 inhibition. Other NSAIDs like ibuprofen and naproxen are better anti-inflammatories but lack antiplatelet activity. Paracetamol lacks anti-inflammatory effects because it cannot penetrate inflammatory sites due to peroxidase protection of COX enzymes. For children with fever, paracetamol is preferred over aspirin to avoid Reye syndrome. The choice of NSAID depends on the clinical indication and patient factors.

Over-the-counter pain relievers are categorized into three main groups: NSAIDs (non-steroidal anti-inflammatory drugs), acetaminophen, and opioids. NSAIDs—including aspirin, ibuprofen, and naproxen—reduce pain, inflammation, and fever by inhibiting COX-1 and COX-2 enzymes, which produce prostaglandins responsible for these symptoms. Aspirin, developed in 1897, is the oldest modern NSAID and uniquely inhibits thromboxane, reducing blood clotting and lowering heart attack or stroke risk when taken regularly in low doses. However, aspirin is linked to Reye’s Syndrome in children during viral infections and can cause gastrointestinal ulcers. Ibuprofen and naproxen, developed later, are safer for children and gentler on the stomach than aspirin but carry increased cardiovascular risks. Acetaminophen (paracetamol) is not an NSAID and lacks anti-inflammatory properties; it reduces pain and fever without affecting prostaglandins in the same way as NSAIDs. It is safe for children and does not cause gastrointestinal bleeding, making it a preferred alternative to aspirin. However, acetaminophen can cause severe liver damage in modest overdoses, and its exact mechanism remains incompletely understood. Opioids like codeine and morphine are potent but not available over the counter in the U.S. due to addiction risks, though codeine is available in small doses with other analgesics in some countries. Aspirin’s origins trace to salicylic acid from willow bark, used since ancient times. Acetaminophen was rediscovered in the 1950s after toxic alternatives (phenacetin, acetanilide) were withdrawn.

Aspirin contains an acetyl group that covalently binds to COX enzyme active sites, irreversibly inactivating them. This irreversible inhibition distinguishes aspirin from other NSAIDs. NSAIDs produce three main therapeutic effects: anti-inflammatory (reducing inflammation by decreasing prostaglandin production), analgesic (relieving pain by reducing prostaglandin-mediated sensitization of pain receptors), and antipyretic (reducing fever by acting on the hypothalamic thermoregulatory center). Acetaminophen lacks significant anti-inflammatory effects but provides analgesic and antipyretic effects. All NSAIDs share similar mechanisms but differ in relative efficacy for different therapeutic effects.

Non-steroidal anti-inflammatory drugs (NSAIDs) are medications that reduce pain, fever, and inflammation by inhibiting cyclooxygenase (COX) enzymes, which block the production of prostaglandins; they are classified into four main categories: non-selective COX inhibitors (such as aspirin and ibuprofen), selective COX-2 inhibitors, other NSAIDs with anti-inflammatory action, and analgesic-antipyretic drugs with anti-inflammatory action (like paracetamol), with the primary mechanism involving the inhibition of arachidonic acid conversion to prostaglandins that mediate pain, fever, and inflammatory responses.

This comprehensive lecture covers three major analgesic classes. Aspirin irreversibly inhibits COX enzymes, reducing prostaglandins and prothrombin; high doses cause mixed acid-base disturbances (respiratory alkalosis from hyperventilation, metabolic acidosis from metabolites) and CNS depression. Critically, aspirin should never be given to children with viral infections due to Reye syndrome risk (1-10% incidence). NSAIDs work by inhibiting COX enzymes, with COX-1 protecting stomach/kidneys and COX-2 mediating inflammation. Selective COX-2 inhibitors like celecoxib were withdrawn due to cardiovascular risks. Acetaminophen works primarily in the CNS for fever/pain relief, with 95% safe metabolism and 5% forming toxic NAPQI detoxified by glutathione. Maximum daily dose is 4 grams for adults. Overdose treatment involves N-acetylcysteine. Key clinical warnings include avoiding aspirin in children, monitoring acetaminophen dosing, and recognizing Reye syndrome symptoms.

Aspirin provides cardioprotection through irreversible COX-1 inhibition in platelets, blocking thromboxane A2 synthesis and preventing arterial thrombosis. Low-dose aspirin (75-162 mg/day) reduces myocardial infarction, stroke, and cardiovascular mortality in high-risk patients. The antiplatelet effect persists for platelet lifespan (~7-10 days) due to irreversible enzyme inhibition. Clinical applications include secondary prevention of cardiovascular events and perioperative management (discontinuation 7 days before surgery). However, aspirin is contraindicated in children with viral infections due to Reye's syndrome risk.

Antiplatelet drugs (aspirin, clopidogrel, prasugrel, ticagrelor) are essential for treating atherosclerotic cardiovascular diseases and acute conditions like myocardial infarction and stroke. Myocardial infarction develops when plaque rupture triggers platelet aggregation. Aspirin, first synthesized by Bayer in the late 19th century, has dose-dependent effects: high doses (500-1000 mg) provide antipyretic and anti-inflammatory actions, while low doses (75-100 mg) exert antiplatelet effects by irreversibly inhibiting COX-1. Aspirin is contraindicated in children due to Reye's syndrome risk. For patients with established atherosclerotic disease, aspirin is indicated with loading doses of 300-500 mg followed by 100 mg daily. In acute coronary syndromes and stent placement, dual antiplatelet therapy (DAPT) combining aspirin with P2Y12 inhibitors (clopidogrel, prasugrel, ticagrelor) is required for 12 months, with shorter durations for high-bleeding-risk patients. Ticagrelor is contraindicated in stroke patients and those over 75 years.

Aspirin irreversibly inhibits COX-1 enzyme in platelets, preventing thromboxane A2 production and platelet activation. Low-dose aspirin (75-100 mg daily) is optimal for antiplatelet therapy. Aspirin is used in stroke prevention and acute coronary syndromes. It is contraindicated in children with viral infections due to Reye's syndrome risk. Higher doses are used for anti-inflammatory effects but not optimal for antiplatelet therapy.

Blood consists of plasma and formed elements (hematocrit 35-45%). Antiplatelet agents (aspirin, clopidogrel) reduce platelet aggregation, not blood viscosity. Aspirin is indicated for stenosing atherosclerosis (≥50% narrowing), post-vascular surgery (stenting, bypass), stroke/TIA, and carotid surgery. Age, hypertension, and varicose veins are NOT indications. Research shows 400 people must take aspirin to prevent one stroke, but this causes 5 fatal hemorrhages. Misconceptions include: 'thick blood' from dehydration, isolated high hemoglobin, and 'blood thinners' affecting plasma levels. Aspirin does not affect hemoglobin unless GI bleeding occurs.

Low-dose aspirin (75-100 mg) is used for antiplatelet therapy to prevent cardiovascular events. At this dose, aspirin irreversibly inhibits COX-1 in platelets, preventing thromboxane A2 production and platelet aggregation. This effect is long-lasting because platelets cannot synthesize new COX-1 enzymes.

Aspirin side effects include GI complications (gastritis, ulcers), insulin resistance (causing type 2 diabetes), aspirin-induced asthma (via leukotriene excess), and ototoxicity (tinnitus, vertigo). Contraindications include bleeding disorders (dengue, chickenpox, hemophilia), G6PD deficiency (causing hemolysis), and pregnancy (teratogenic). Reye's syndrome is a serious complication of aspirin use in viral infections, characterized by liver damage and encephalopathy. Pathophysiology involves impaired urea cycle, ammonia accumulation, and blood-brain barrier crossing. Clinical features include GI upset, behavioral changes, jaundice, asterixis, and coma. Diagnosis requires liver biopsy, elevated ammonia (10-80 mcg/dL), and BUN (8-25 mg/dL). Management includes aspirin discontinuation, low-protein diet, lactulose, and neomycin. Prevention involves avoiding aspirin in viral infections.

Aspirin has two major side effects: (1) It inhibits prostaglandins that protect the stomach lining, causing ulcers and bleeding; (2) It prevents platelet clumping, preventing heart attacks but also causing dangerous internal bleeding. Most significantly, Reye's syndrome was discovered in the 20th century—children with viral infections taking aspirin entered coma with liver failure and brain swelling. Once the connection was established, strict prohibitions were put in place against giving aspirin to children with viral infections.

Aspirin irreversibly inhibits COX enzymes, preventing prostaglandin and thromboxane A2 production. This provides analgesia by reducing brain prostaglandins, antipyresis by reducing hypothalamic prostaglandins, and anti-inflammatory effects by reducing prostaglandin production. However, aspirin also inhibits protective gastric prostaglandins, causing ulcers. High-dose causes acute ulcers, chronic use causes chronic ulcers. Mild hepatotoxicity may occur. Critically, aspirin is contraindicated in children under 12 with viral infections due to Reye syndrome, a fatal condition causing liver damage and encephalopathy. Prevention requires avoiding aspirin in this population and using alternative antipyretics like acetaminophen.

Aspirin inhibits COX-1, which also produces protective gastric mucus and aids cell turnover, leading to dyspepsia, upper abdominal pain, peptic ulcers, and potentially gastrointestinal bleeding. Higher doses (325 mg) additionally inhibit COX-2, causing tinnitus. Critically, aspirin should never be given to children or teenagers due to Reye's syndrome risk—a rare but serious condition involving brain swelling and liver damage, particularly during recovery from chickenpox or flu. These side effects demonstrate the importance of appropriate dosing and patient selection.

Aspirin uniquely absorbs primarily in the stomach (50%+), unlike most drugs absorbed mainly in intestines. This explains why enteric coating exists—not to prevent ulcers (since damage occurs after absorption), but possibly to make absorption more predictable. Aspirin should never be given to children under 18 due to Reye's syndrome risk, especially during viral illnesses like chickenpox or flu. The video clarifies that aspirin's liquid formulation doesn't exist because aspirin degrades rapidly in water, requiring special effervescent tablets instead.