Why Can Aspirin Be Absorbed through Your Stomach, but Lidocaine Cannot?

Chemical Formula: C₉H₈O₄ (Acetylsalicylic Acid)

pKa: 3.5 - This low pKa means aspirin readily gives up protons in basic conditions

Mechanism: Irreversibly inhibits COX enzymes, reducing prostaglandin synthesis

Absorption Key: As a weak acid (pKa 3.5), aspirin stays mostly unionized in the acidic stomach (pH 1.5), allowing some absorption across cell membranes and some to undergo hydolysis. Most aspirin absorption happens in the small intestine, where the much larger surface area (200+ m²) overcomes the increased ionization that occurs at higher pH levels.

Clinical Insight: This is why aspirin can cause stomach irritation - it's absorbed right where it can damage the gastric lining!