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An oral drug may be available in several dosage forms including the more common tablets and soft/hard gel capsules. A tablet comprises of the active pharmaceutical ingredient (API) and excipients compacted together and coated. Excipients include binders, flow‐aids, lubricants, preservatives, solubilizing, and flavoring agents. The release from these dosage forms and the dissolution of the drug may limit the rate of absorption.

The oral route is noninvasive and thus the most convenient route of drug administration. However, the bioavailability of many oral drugs is limited by drug solubility, dissolution, membrane permeability, or gut metabolism. Special formulations may therefore be needed to enhance drug exposure to the target tissue and control the exposure profile of a drug through the development of enabling and controlled‐release formulations respectively.

During drug development, the performance of different formulations, dosage forms, routes and different conditions of disease, fed/fasted states etc., may be compared even without IV PK, by determining the relative bioavailability, defined by Equation 1.38:

(1.38)

Assuming that the IV clearance (CL_IV ) is same in both conditions, Equation 1.38 reduces to

(1.39)

To ensure equivalent performance of different batches of drug formulations, regulatory agencies require bioequivalence studies to be performed. To establish bioequivalence, the 90% confidence interval about the geometric mean test/reference ratios for both AUC and C_max must fall within the bioequivalence range, which is 80–125%.