The amount of the metabolite formed after drug administration depends on the dose of the drug and f_m. If f_m is equal to unity (1) for a particular metabolite, this means that the entire dose of the parent drug is converted to that metabolite.

Drugs that undergo parallel metabolism to more than one metabolite have different f_m values for each metabolite. The sum of these f_m values should not be more than one, but it does not have to be equal to one (when the drug elimination involves a pathway other than metabolism).

This fraction should be dose independent when drug elimination follows first-order kinetics. Also, this fraction should not be different for a particular drug in a particular patient after different routes of administrations, if the metabolite is not formed during drug absorption after extravascular administration.

The first-order elimination rate constant of the metabolite is the rate constant responsible for the elimination of the metabolite from the body. This rate constant is dependent on the clearance and the volume of distribution of the metabolite. (CL_T(m)) / Vd_(m) = k_(m))

The elimination rate constant of the metabolite is the same when the metabolite is formed in vivo after the parent drug administration and after administration of the preformed metabolite.

This constant represents the metabolite elimination through all routes of metabolite elimination including secondary metabolism and renal excretion.

The first-order elimination rate constant of the metabolite has units of time ^-1.

The first-order elimination rate constant is the rate constant for the elimination of the drug from the body. The elimination rate constant and the half life are dependent on the total body clearance and the volume of distribution of the drug.

This rate constant represents drug elimination through all routes of drug elimination such as metabolism, renal excretion, and other routes.

The first-order elimination rate constant has units of time^-1.