Principles of Drug Action

Principles of Drug Action

A drug passes through three phases in order to achieve its intended purpose. These stages include the drug administration phase, pharmacokinetic phase, and pharmacodynamic phase. These phases form a conceptual framework that explains the drug’s course of action from the administration,  Absorption, effects, and finally, how it is excreted from the body.

Pharmaceutical Phase

Approximately 80% of drugs are taken by mouth. The pharmaceutical phase is the initial stage where the drug is ingested. The drug must be converted into a solution form in the Gastrointestinal tract for them to be absorbed. Therefore, drugs that are taken in the solid form, whether tablets or capsules, must be broken down into tiny particles, which then dissolves into a solution form in a process known as dissolution. Drugs ingested in a liquid state are easily absorbed since they are already in solution form.

Pharmacokinetic Phase

Pharmacokinetic is the process of drug movement from absorption, metabolism, distribution, and excretion. After being dissolved, the drug particles are absorbed into the surface of the small intestines. The Gi tract is made of lipid fats; therefore, drugs that are soluble in lipids dissolve quickly as they pass through the tract. Drugs that are administered through other routes such as ear drops and eye drops are absorbed directly hence do not pass the GI tract or the liver.

In the liver, the drug can be metabolized into various forms. Some medications are converted into more active forms, which are then distributed around the body through body fluids, often through the blood. Others are metabolized into less active states, which is then excreted from the body through the urine or feces. Factors that may affect the absorption process include blood flow, pain, stress and depression, and PH levels. Some drugs are better absorbed in an alkaline environment, while others prefer an acidic environment.


Pharmacodynamics is the third and the last phase in the drug action framework. It is concerned with the relationship between the drug concentration and the effects it has on a patient. Drugs work by interacting with biological structures located inside the cells, such as receptors and enzymes, to produce desired outcomes. Therefore, the concentration of a drug at these sites determines the length and the intensity of the effects at the site of action.

However, determining the intensity of a drug at the site of action is not easy. It calls for a deeper understanding of the interaction of that particular drug with the target. Most drug-producing companies, therefore, prefer manufacturing drugs that interact with specific receptors in an effort to reduce possible side effects.

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