Two or more drugs given at the same time may exert their effects independently or may interact. The potential for interaction increases the greater the number of drugs employed. Most patients admitted to an intensive care unit will be on more than one drug.

Drugs interactions can be grouped into three principal subdivisions: pharmacokinetic, pharmacodynamic and pharmaceutical.

1. • Pharmacokinetic interactions are those that include transport to and from the receptor site and consist of absorption, distribution, metabolism and excretion.

2. • Pharmacodynamic interactions occur between drugs which have similar or antagonistic pharmacological effects or side-effects. This may be due to competition at receptor sites or can occur between drugs acting on the same physiological system. They are usually predictable from a knowledge of the pharmacology of the interacting drugs.

3. • Pharmaceutical interactions are physical, and chemical incompatibilities may result in loss of potency, increase in toxicity or other adverse effects. The solutions may become opalescent or precipitation may occur, but in many instances there is no visual indication of incompatibility. Precipitation reactions may occur as a result of pH, concentration changes or ‘salting-out’ effects.