**8. Systemic availability of drugs**

A drug will reach systemic arterial circulation only if it is absorbed from the GIT and if it escapes metabolism in the gut, liver, and lungs. When the concentration of the drug in plasma is measured at specified time intervals, it is possible to construct concentration versus time graph and hence be able to determine the extent of drug availability as shown in Figure 4.

The availability depends on both the extent of absorption and the extent of presystemic metabo‐ lism and comprises three aspects; Peak concentration (Cmax), Time taken to reach the peak (Tmax) and area under the curve (AUC)as shown in Figure 4. The Cmax and Tmax are measures of the rate of availability while AUC is a measure of the extent of availability (i.e. proportion of the admin‐ istered drug which reaches systemic circulation intact). For the three curves shown for the for‐ mulations a, b, and c; the AUC is the same, but the rate of availability is different in each case; a, has the lowest rate of availability followed by b, while c has the highest rate of availability.

**Figure 4.** Plasma concentration versus time curves for different drug formulations

The speed at which a particular drug is needed to reach the site of action will determine the type of formulation to use. Drugs with the same relative bioavailability and can be used to treat the same condition using either the same routes or dosages are known as bioequivalent drugs.
