Bioavailability and Bioequivalence

When giving an intravenous injection (IV) directly into the bloodstream, the bioavailability is by definition 100%. Figure 1 shows the bioavailability profile of medicines given IV. This is shown as concentration over time in hours.

Figure 1: The percentage of active substance in the body or bioavailability, after direct injection into the blood stream and observed over a period of 15 hours. The area under the curve (AUC), is shaded. T_max is the time when the highest concentration of the medicine is found in the blood, C_max is the maximum concentration.

After injection, an API reaches the target site after a complex journey in the bloodstream and body.Immediately after injection, the bioavailability will be 100%  (seen on the y-axis in Figure 1 as C_max and on the x-axis at T_max). So, if 75 milligram (mg) of active substance is injected into the bloodstream, then 100% corresponds to 75 mg active substance. The concentration of the API in the body will then decrease over time (Figure 1) because, a fraction of it will be metabolised or excreted.