[The pharmacokinetic process: is the drug getting to its site of action? Drug absorption]

The pharmacokinetic process is concerned with the absorption, distribution, and elimination (by metabolism and excretion) of drugs. It is evident that drug molecules have to pass many structural and metabolic barriers. The concentration of a drug at its site of action must have the blood concentration as one of its determinants. Only the free form of the drug is susceptible of management by the body Drug bioavailability and the factors that may affect it are influencing the choice of an optimal route of drug administration and the pharmaceutical form as well as bioequivalence between different preparations. There are three kinds of factors that may affect drug bioavailability: drug-related, patient-related and environmental-related factors. A reduction of the speed of absorption is leading to diminished pharmacological after single oral doses of the drug but it is not affecting the steady-state concentration of the drug or drug effects. On the contrary Tmax is not affected by those factors that are reducing the amount of available drug in the body through drug absorption impairment. However, they may cause therapeutic failure due to low steady-state concentration of the drug during long-term treatment.