The steady state is the condition when the rate of drug administration is equal to the rate of drug elimination.

When the drug is administered as a constant rate IV infusion, there will be one steady state concentration as long as the rate of administration and the rate of elimination do not change.

When drug administration involves repeated doses, the drug concentration-time profile during each of the dosing intervals ( τ ) will be identical at steady state. This means that the maximum, the minimum, and the average concentrations during each dosing interval will be similar at steady state.

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.

The total body clearance is the volume of the plasma or blood that is completely cleared from the drug per unit time. It has units of vloume/time.

The CL_T is the sum of all organ clearances (i.e. renal clearance, metabolic clearance, and other clearances).

The CL_T for a drug is constant within a patient (dose and concentration independent) when the elimination process follows first-order kinetics.

When the drug elimination process follows zero-order kinetics the CL_T is concentration dependent. The CL_T is lower at higher drug concentration.

The half life is the time required for the amount of the drug or the plasma concentration of the drug to decrease by 50% . The half life has units of time.

The half life of a drug is constant within a patient (dose and concentration independent) when the drug elimination process follows first-order kinetics.

However, the half life is concentration dependent when the elimination process is saturable (zero-order). The half life is usually longer at higher concentration.

The half life is a very important parameter, because it indicates how fast the drug is eliminated from the body.