There are many factors that affect the bioavailability, including the size of drug particles, crystal form, compactness of fillers, excipients and production technology. Bioavailability is an index used to evaluate the absorption degree of preparation.

Other factors are: ① Gastrointestinal catabolism; ② First-pass effect of liver; ③ the influence of nonlinear characteristics; ④ The influence of experimental animals; ⑤ The influence of age, disease and food.

Bioavailability (F) refers to the relative amount of drugs absorbed into systemic blood circulation after administration through extravascular route.

F=(A/D)X 100%。

A is the total amount of drugs in the body, and D is the dose.