Medicines may be administered to the patient in a variety of ways, but the desired therapeutic effect will be achieved only if the pharmacologically active substance reaches its site of action (the target cells in the body) in a concentration sufficient for the appropriate effect and remains there for an adequate period of time before being excreted. Thus, to produce its characteristic effects, a drug must undergo a process of movement from the site of application into the extracellular compartment of the body and be present in appropriate concentrations at its sites of action. Absorption may therefore be defined as the sum of all processes that a drug substance may undergo after its administration before reaching the systemic circulation. Consequently, it is evident that the concentration of active drug attained depends primarily upon the extent and rate of absorption. The extent (completeness) of absorption into the systemic circulation is sometimes defined by another parameter, designated as bioavailability. Generally, the term is used to indicate the fractional extent to which a dose of drug reaches its site of action, or a biological fluid from which the drug has access to its site of action. The amount of drug absorbed is determined by measuring the plasma concentration at intervals after dosing and integrating by estimating the area under the plasma concentration versus time .Bioavailability may vary not only between different drugs and different pharmaceutical formulations of the same drug, but also from one individual to another, depending on various factors, described in a following subsection.

The rate of absorption, expressed as the time to peak plasma concentration (Tmax), determines the onset of pharmacological action, and also influences the intensity and sometimes the duration of drug action, and is important in addition to the extent (completeness) of absorption. Moreover, we can define the concept of absolute bioavailability as the percentage of the drug substance contained in a defined drug formulation that enters the systemic circulation intact after initial administration of the product via the selected route. Nevertheless, it is noted that while the absolute bioavailability of two drugs may be the same (as indicated by the same AUC) the kinetics may be very different (e.g. one may have a much higher peak plasma concentration than the other, but a shorter duration).

 As already mentioned, drugs may be administered by many different routes, the choice of which depends upon both convenience and necessity. Under the circumstances, it is evident that knowing the advantages and disadvantages of the different routes of administration is of primary importance. The most common, generally safe, convenient for access to the systemic circulation and most economical method of drug administration is the oral route, applicable for achieving either local or systemic effects. A number of recent reviews treat various aspects that are relevant to drug administration via this route. For example, an account has been given of the structure of oral mucosa and the factors that affect drug oral mucosal absorption and drug formulation.

Best Regards,
Nancy Ella
Editor-In-Charge
Drug Designing: Open Access