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Summary of Pharmacology :-

Pharmacology is the study of the actions, mechanisms, uses and adverse effects of drugs.
A drug is any natural or synthetic substance that alters the physiological state of a living organism.

Drugs can be divided into two groups :-
  1. Medicinal drugs: substances used for the prevention, treatment and diagnosis of disease.
  2. Nonmedicinal (social) drugs: substances used for recreational purposes. These drugs include illegal substances such as cannabis, heroin and cocaine, as well as everyday substances such as caffeine, nicotine and alcohol.

How do drugs work ?
Most drugs produce their effects by targeting specific cellular macromolecules, often proteins. The majority act as receptors in cell membranes, but they can also inhibit enzymes and transporter molecules. Some drugs directly interact with molecular targets found in pathogens. For example, β-lactam antibiotics are bactericidal, acting by interfering with bacterial cell wall synthesis.

Certain drugs do not have conventional targets. For example, succimer is a chelating drug that is used to treat heavy metal poisoning. It binds to metals, rendering them inactive and more readily excretable. Such drugs work by means of their physicochemical properties and are said to have a nonspecific mechanism of action. For this reason, these drugs must be given in much higher doses than the more specific drugs. Another example would be antacids used to reduce the effect of excessive acid secretion in the stomach.

Most drugs produce their effects by acting on specific protein molecules called receptors. Receptors respond to endogenous chemicals in the body that are either synaptic transmitter substances (e.g. ACh, noradrenaline) or hormones (endocrine, e.g. insulin; or local mediators, e.g. histamine).

These chemicals or drugs are classed in two ways.
  1. Agonists : Activate receptors and produce a subsequent response. 
  2. Antagonists : Associate with receptors but do not cause activation. Antagonists reduce the chance of transmitters or agonists binding to the receptor and thereby oppose their action by effectively diluting or removing the receptors from the system.

Drug Absorption
Bioavailability takes into account both absorption and metabolism and describes the proportion of the drug that passes into the systemic circulation. This will be 100% after an intravenous injection, but following oral administration, it will depend on the physiochemical characterizations of the drug, the individual and the circumstances under which the drug is given.

Drugs must cross membranes to enter cells or to transfer between body compartments; therefore drug absorption will be affected by both physiochemical and physiological factors.