Endogenous or exogenous substances which inhibit the normal growth of human and animal cells or micro-organisms, as distinguished from those affecting plant growth (plant growth regulators).
A substance that hampers the development of an organism is referred to as an inhibitor. This all-encompassing definition encompasses the action of certain fungicides, as well as auto-inhibition by an organism. Unless qualified and the dosage is taken into account, the term is arduous to define. In most instances, the degree and type of response are dependent on the concentration of inhibitors present. If present in high concentrations, substances can be toxic and irreversibly inhibit growth, resulting in cell death. However, in low concentrations, they can act as a growth retardant that can be reversed by returning the organism to an inhibitor-free environment. The type of response is also determined, to some extent, by the duration of exposure to the inhibitor. At very low concentrations, certain growth inhibitors function as growth stimulants, such as coumarin.