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The microsomal enzyme system is responsible for the metabolism of pollutants, toxic agents and drugs. With regards to drug metabolism, the activity of the constituent microsomal enzymes results in the reduction of pharmacological and toxicological activity through conversion of hydrophilic (water soluble) compounds to allow urinary excretion. Microsomal enzymes oxidize drugs and steroid hormones in reactions that require adenine nucleotide diphosphate (NADPH). Reversible inhibition reduces enzyme activity through reversible interaction. A covalent bond between the inhibitor and the enzyme can promote the destruction of essential functional groups of the enzyme. Enzyme induction and inhibition are problematic in drug polytherapy. Often the lack of effect of a drug or the side effects that a certain drug exhibits are problems of interaction of drugs with each other in that individually they inhibit or stimulate enzyme activity.