Drug bioavailability refers to how quickly and how much a drug is absorbed in the body. Factors like the drug's properties, how it's made, and the patient's gut health can affect this. Eating food or taking other medications can also change how well a drug is absorbed. Scientists can measure bioavailability by comparing how much of a drug is absorbed when taken by mouth versus injected. They can also use a special method involving stable isotopes to study this. By using specific equations, researchers can calculate the rate and extent of drug absorption in the body.