Low doses of biologically active substances: effects, possible mechanisms, and features.

Some substances can produce a response in biological systems at extraordinarily low concentrations. Often called the 'low' or 'small' dose effect, the meaning or definition of what is meant by 'low' in this context is discussed. For the purposes of this article the expression is taken to be a concentration substantially below the equilibrium dissociation constant of the effector-target complex, but extremely low concentrations (<10(-19) m) are excluded from consideration.