Drugs exert their observed effects in a multitude of different ways. They may be physicochemical, pharmacodynamic or pharmacokinetic interactions with biochemical and physiological systems of the body.

The main, intended action of a drug may not be the only effect that substance has on the body, and there may be multiple modes of action. For example, many drugs interact with more than one type of receptor and some may alter the pharmacokinetics of co-administered drugs by enzyme induction or inhibition.

Physicochemical mechanisms

These mechanisms are generally non-specific, and depend on the physicochemical properties of a drug including its molecular size and shape, the degree of ionisation and pKa of its constituent groups and the lipid and water solubility of the drug.

Non-specific physicochemical mechanisms of action include:

• Charge neutralisation (pH effects)

• Osmotic effects

• Adsorption

• Chelation

Charge neutralisation

This mode of action is typified by the action of the antacid drugs. Sodium citrate is the salt of a weak acid, and combination in the stomach with hydrochloric acid, a strong acid, produces sodium chloride and citric acid, a weaker acid, thus reducing intragastric pH. Calcium bicarbonate is also an effective antacid, but the reaction produces carbon dioxide, which can cause abdominal distension and flatulence. To avoid this problem sodium citrate is preferred preoperatively to reduce the risk of aspiration-induced lung damage in high-risk patients requiring general anaesthesia. Sodium citrate should not be used long-term because of the high sodium load; antacid drugs for prolonged use ideally should not be absorbed.