Bioavailability is the amount of a therapeutically active substance that has reached the systemic bloodstream and has become available at the place of application of its action.

Bioavailability is also one of the essential parameters used in pharmacokinetics, taken into account when calculating the dosage regimen for routes of administration of drugs other than intravenous.

Bioavailability can be as follows:
 absolute
 relative

Absolute bioavailability (f) is the portion of the dose of the drug (in %) that reached the systemic blood flow after extravascular administration.
Total bioavailability is a part of the oral dose of the drug that has reached the systemic bloodstream unchanged and in the form of metabolites formed during absorption as a result of so
the so-called presystemic metabolism, or «primary passage effect».

Relative bioavailability is the PPK of a certain drug, comparable to another formulation form of the same drug, accepted as standard, or introduced into the body in another way. When the standard represents an intravenously administered drug, we are dealing with absolute bioavailability. To determine the relative bioavailability
data on the level of the drug in the blood or its excretion in the urine after a single or multiple administration can be used.

Factors determining bioavailability.

The absolute bioavailability of a certain drug administered by an extravascular route is usually less
1 (F ‹ 1,0). Various physiological factors reduce the bioavailability of drugs before they enter the systemic
blood flow.

These factors include:

  • physical properties of the drug, in particular, hydrophobicity, degree of dissociation into ions, solubility),
  • dosage forms of the drug (immediate release, use of excipients, production methods, modified — delayed, prolonged or prolonged release,
  • is the drug administered on an empty stomach or after a meal,
  • differences during the day,
  • the rate of gastric emptying,
  • induction/inhibition by other drugs or food:
    a) interaction with other medications (antacids, alcohol, nicotine),
    b) interaction with individual food products (grapefruit juice, pomelo, cranberry juice).
  • carrier proteins, a substrate for a carrier protein (e.g., P-glycoprotein).
  • the state of the gastrointestinal tract, its function and morphology.