**3.1 Principles of drug actions**

There are eight main drug actions and these are:

**Stimulation:** Through direct receptor agonism and downstream effect e.g. adrenaline stimulates heart, pilocarpine stimuates salivary glands. However, excessive stimulation is often followed by depression of that function e.g. high dose of picrotoxin, a CNS stimulant, produces convulsions followed by coma and respiratory depression.

**Depression:** Through direct receptor agonism and down stream effect e.g. barbiturates depress CNS while quinines depresses the heart. The action of this mechanism is selective.

**Blocking/Antagonizing action:** The drugs binds the receptor but does not activate it.

**Stabilizing action:** In this case, the drugs seem to act neither as a stimulant nor as a depressant but to stabilize general receptor activation like buprenorphine in opioid dependence or aripiprazole in schizophrenia.

**Replacement:** Refers to the use of natural metabolites including hormones and vitamins in deficiency stages e.g. levodopa in Parkinsonism, insulin in diabetes mellitus, iron in anemia and oestrogen replacement in women of menopausal age.

**Direct beneficial chemical reaction:** As in use of antioxidants like Vitamins C,E and *B*carotene for free radical scavenging

**Cytotoxic action:** Selective cytotoxic action for parasite, bacterial or cancer cells, attenuating them without significantly affecting the host cells e.g. use of antibiotics like penicillin, zidovudin and cyclophosphamide

**Irritation:** A none selective often noxious effect applicable to less specialized cells, for example the epithelial, connective tissue cells). Mild irritation may stimulate associated function e.g. bitters increase salivary and gastric secretions which results in increased blood flow to the site. However, strong irritation may result in inflammation, corrosion, necrosis and morphological damage with resultant diminution or loss of function. Therefore caution should be exercised in the administration because of tendency of excessive ingestion.
