In enzyme kinetics V_max is the maximum rate of the enzymatic reaction. For drug metabolism in vivo, V_max is the maximum rate of drug metabolism by a specific pathway.V_max has units of amount per time and can also have units of concentration per time.

The maximum rate of drug metabolism is dependent on the amount of enzyme available for drug metabolism via a specific pathway. Enzyme induction results in increasing the amount of the enzyme and V_max.

Diseases that affect the liver (the primary organ for drug metabolism) may result in decreasing the metabolic capacity for metabolic pathways and decreases V_max. Also, V_max may depend on the size of the patient. Children may have lower V_max compared to adults.

It is the Michaelis-Menten constant. It is a qualitative characteristic of how an enzyme interacts with the substrate and is independent of the enzyme concentration. K_m is equal to the substrate conc when the reaction rate is half its maximum value (1/2 V_max).

K_m has units of concentrations (amount/volume).

Conditions such as the presence of a competitive inhibitor can result in increasing K_m. This is because at any given drug conc the rate of drug metabolism is slower in presence of a competitive inhibitor compared to the rate in absence of the inhibitor.