While medical doctors frequently prescribe medicine to their patients, it is up to the pharmacist, the last dispenser of the medicine to make sure that the patient receives the appropriate dosage. The design of a dosage regimen depends on an understanding of the drug use process (DUP), which asks ‘Is this patient suffering from a drug-related problem?’
Pharmacokinetics provides a mathematical basis to assess the time course of drugs and their effects in the body. It quantifies the following four processes: Absorption, Distribution, Metabolism, and Excretion.
There are a variety of available techniques for representing the pharmacokinetics of a drug. The most usual takes the view of the body consisting of compartments between which the drug moves and from which elimination occurs. The transfer of drugs between these compartments occurs by rate constants.
Some of the rate constants are determined by:
- The Volume of distribution (Vd); this connects the administered dose of medicine with the actual initial concentration.
- Clearance (Cl); this is the removal of the drug, which normally takes place through the liver.
Half-life (t1/2); this is the time taken for a drug concentration to be eliminated in the plasma by a factor of 50%.
These factors are important for pharmacists to know and keep in mind when dispensing prescription medicines. Normally, at the pharmacy office, a computer system has programs that will allow the pharmacist to get the ideal dosage for the patient. However, in most cases the operation may be slow, or may involve having to maneuver though multiple screens in order to access the relevant information.
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