Bioavailability is a measure of how easily a substance can be absorbed by the body. In pharmacology, for example, it refers to how quickly a drug enters the circulatory system and reaches the desired area, so that it can take effect. The bioavailability of a drug has important therapeutic implications, because it is the major factor determining the drug dosage for different routes of administration.

By increasing the absorption of active ingredients into the body through improving their permeability through the skin (topical), the lining of the mouth (buccal or sublingual), or the lining of the stomach or gut (enteric) active ingredients can be used at lower doses, reduce side effects and be much more effective. All the routes of drug administration need to be understood in terms of their implications for the effectiveness of the drug therapy and the patient’s experience of  treatment.