In enzyme kinetics, the enzyme-catalysed reactions are some biochemical reactions in which enzymes speed up the conversion of substrate into product by lowering their activation energy. In these reactions, some of the chemical species exhibit different intrinsic time scales and tend to reach an equilibrium state much quicker than the other species. A commonly used mechanism to simplify the description of the dynamics of such biological systems is the quasi-steady-state approximation. In this paper, we investigate the validity of the standard quasi-steady-state approximation for the Michaelis-Menten reaction model with inhibition and a constant substrate input. Necessary and sufficient conditions for the validity of these assumptions were derived and were shown to be dependent, among others, on the substrate input. The validity conditions are verified numerically using the classical Runge-Kutta method.