Two of the most widely used groups of drugs in medical practice are the non-steroidal anti-inflammatory agents and the steroids. Both act by modulating the conversion of essential fatty acids to prostaglandins, leukotrienes and related substances. The actions of these drugs are therefore likely to be modified by variations in the levels of substrates, notably arachidonic acid and dihomogammalinolenic acid, available for metabolism by lipoxygenase and cyclo-oxygenase enzymes. Yet most doctors who use the drugs and many scientists who carry out research on them seem unaware of the factors which determine the concentrations of the substrate essential fatty acids. This paper reviews in detail the metabolism of essential fatty acids and the interactions between nutrient intake and subsequent metabolism which determine the concentrations of the individual fatty acids. It is concluded that the efficacy of drug therapy as far as the steroids and the non-steroidal anti-inflammatory drugs are concerned could be substantially enhanced by greater knowledge of the factors which determine the availability of substrates to the key enzymes.