Bioavailability is the fraction of an administered drug dose that reaches the systemic circulation in an unchanged, pharmacologically active form. It is expressed as a percentage: a drug with 100% bioavailability delivers its entire dose to the bloodstream; a drug with 50% bioavailability delivers half.
Intravenous administration has 100% bioavailability by definition — the drug is placed directly into the bloodstream. Every other route of administration has lower bioavailability because the drug must cross biological barriers and may be degraded or metabolized before reaching systemic circulation.
Factors affecting oral bioavailability
Oral administration — the most common route — subjects drugs to several barriers:
- Dissolution and absorption — the drug must dissolve in the gastrointestinal fluid and cross the intestinal wall. Drugs with poor water solubility, large molecular size, or vulnerability to acid degradation have reduced absorption. Food, gastric pH, and gut motility all influence this step.
- First-pass metabolism — after crossing the intestinal wall, orally administered drugs enter the portal vein and pass through the liver before reaching systemic circulation. The liver’s CYP enzymes may metabolize a significant fraction of the drug on this first pass, dramatically reducing bioavailability. This is why some drugs (like nitroglycerin, which is almost entirely destroyed by first-pass metabolism) cannot be given orally and must be administered sublingually, transdermally, or intravenously.
- Intestinal metabolism — the gut wall itself contains metabolic enzymes that can degrade drugs before they reach the portal circulation.
- Efflux transporters — membrane proteins (like P-glycoprotein) in the intestinal wall actively pump drugs back into the gut lumen, reducing the fraction absorbed.
Clinical significance
Bioavailability matters because it determines the relationship between the dose administered and the dose that actually reaches the drug’s target. When switching between routes of administration (e.g., from IV to oral), the clinician must adjust the dose to account for bioavailability differences. A drug with 50% oral bioavailability requires twice the oral dose to achieve the same plasma concentration as an IV dose.
Bioavailability also explains variability between patients and between different formulations of the same drug. Generic drugs are required to demonstrate “bioequivalence” — similar bioavailability to the branded version — but small differences can matter for drugs with narrow therapeutic indices.
In harm reduction contexts, bioavailability explains why route of administration dramatically affects both the effect and the risk of a substance. Smoking or injecting a drug bypasses first-pass metabolism, producing higher peak concentrations and faster onset than oral use — which explains both the more intense subjective effect and the higher overdose risk of these routes.
Related terms
- Pharmacokinetics — the broader framework of which bioavailability is one component
- Half-Life — how long the drug remains in the body once it has reached circulation
- Therapeutic Index — the margin of safety that bioavailability variability can narrow