More information about the oral route of drug administration:
Oral (p.o.) Ingestion
a. Convenient route for administration of solid as well as liquid formulations.b. Additional variables which may influence rate and extent of absorption include disintegration and dissolution of solids, acidity of gastric contents, gastric emptying rate, intraluminal and mucosal biotransformation by host or bacterial enzymes, dietary contents, and presence of other drugs.
c. First-pass effect: absorbed drug passes via portal circulation through liver which may clear substantial fraction and thus decrease bioavailability (percent of
dose which reaches the systemic circulation).
Source of text: Principles of Pharmacology Course, MIT OpenCourseWare. Pharmacokinetics