Abstract: The pharmaceutical compositions of the present invention comprise orally administerable dosage forms that use effervescence as a penetration enhancer for drugs known, or suspected, of having poor bioavailability. Effervescence can occur in the stomach, once the tablet or other dosage form is ingested. In addition to effervescence in the stomach, or as alternative technique, by the use of appropriate coatings and other techniques, the effervescence can occur in other parts of the gastrointestinal tract, including, but not limited to, the esophagus, duodenum, and colon. The site of effervescence and drug release is chosen to correspond with the segment of the gastrointestinal tract displaying maximal absorption of the formulated drug, or to gain some other therapeutic advantage.