Absorption & Bioavailability

Two drugs with identical doses can deliver wildly different amounts to the bloodstream. The difference is bioavailability — and it explains why route, timing, and formulation change everything.

Absorption is the movement of a drug from its administration site into systemic circulation. Bioavailability is the fraction of the administered dose that reaches circulation in active form — IV drugs have 100% bioavailability by definition because they bypass absorption entirely. Oral drugs must survive the GI environment and first-pass metabolism in the liver before reaching the bloodstream, which often reduces bioavailability significantly. Several factors alter oral absorption: gastric pH (antacids raise pH, reducing absorption of drugs needing an acidic environment), GI motility (diarrhea speeds transit and decreases absorption; constipation may increase it for some drugs), food interactions (high-fat meals delay absorption of some drugs but enhance others like griseofulvin), blood flow to the GI tract (shock states dramatically reduce absorption), and drug formulation (enteric coatings, sustained-release tablets, and crushability all matter). Sublingual and rectal routes partially bypass first-pass metabolism, yielding higher bioavailability than oral for the same drug. This is why sublingual nitroglycerin works in minutes while an oral form would be largely inactivated by the liver.

Key Distinctions

Don't confuse absorption (getting into the blood) with distribution (moving from blood to tissues) — absorption ends when the drug enters systemic circulation. Students often equate bioavailability with potency; bioavailability is about how much arrives, not how strongly it acts. Crushing an extended-release tablet doesn't just change timing — it destroys the absorption-control mechanism and can deliver a toxic bolus dose.

Clinical Pearl

Think of first-pass metabolism as a tollbooth: every oral drug pays a tax at the liver before reaching the highway. IV drugs take the express lane — no toll, 100% through.