Principles of Drug Actionare important to be understood. Drugs do not create new functions to any system, cell or organ. They can only modify the inherent functions.

Principles of Drug Action shows that they produce only a quantitative and not a qualitative change in the functions of the target organ.

The basic types of drug action that are based on Principles of Drug Action are:

1. Stimulation: Selective enhancement in the activity of specialized cells is called stimulation, e.g. caffeine stimulates central nervous system.

2. Depression: Selective diminution in the activity of specialized cells is called depression, e.g. barbiturates depress central nervous system.

3. Irritation: This indicates a non-selective and often noxious effect. It is particularly applied to less specialized cells such as epithelium, connective tissue, etc. Such drugs produce effects on the growth, nutrition, and morphology of living tissues. Mild irritation stimulates function, e.g. bitters increase salivary and gastric secretions. On the other hand, strong irritation leads to inflammation, corrosion, necrosis and morphological damage. Finally, this may result in diminution or loss of function.

4. Use of natural metabolites, hormones or their congeners in deficiency states (e.g. levodopa in parkinsonism, insulin in diabetes mellitus) is called replacement therapy.

5. As anti-infective agents: Drugs are used for prevention, arrest and eradication of infection. They act specifically on the causative organisms and host cells are least affected, e.g. penicillin.

6. Modification of immune status is one of the Principles of Drug Action: Sera, vaccines, and certain other drugs act by altering the immune status.