After oral administration the fraction of drug reaching the systemic circulation in unchanged form is:

(Ref: KD Tripathi, 7th ed. pg. 16)BIOAVAILABILITYBioavailability is a measure of the fraction of administered dose of a drug that reaches the systemic circulation in the unchanged form.Bioavailability of drug injected IV is 100%, but isfrequently lower after oral ingestion because--#The drug may be incompletely absorbed.#The absorbed drug may undergo first pass metabolism in the intestinal wall/liver or be excreted in bile.