Another facet of pharmaceutical research that has attracted wide medical attention is the “availability” to the body (bioavailability) of various dosage forms of drugs. Exact methods of determining levels of drugs in blood and organs have revealed that slight changes in the mode of manufacture or the incorporation of a small amount of inert ingredient in a tablet may diminish or completely prevent its absorption from the gastrointestinal tract, thus nullifying the action of the drug. Ingenious methods have been devised to test the bioavailability of dosage forms. Although such in vitro, or test-tube, methods are useful and indicative, the ultimate test of bioavailability is the patient’s response to the dosage form of the drug.