Intravenous injection IV bolus

The plasma concentration C of a drug in the system is related to the total amount of the drug in the system D by the relationship  

Drug Vd (litres kg ) hi2 (hours) Drug Vd (litres kg ) ha (hours) 

The rate of elimination is an important characteristic of a drug. Too rapid an elimination necessitates frequent repeated administration of the drug if its concentration is to reach its therapeutic window. Conversely, too slow an elimination could result in the accumulation of the drug in the patient, which might give an increased risk of toxic effects. Most drug eliminations follow first order kinetics (equations (8.1) and (8.2)), no matter how the drug is administered, but there are some notable exceptions, such as ethanol which exhibits zero order kinetics where  

However, the order of elimination processes may change depending on the biological situation, for example, drug concentration increasing to a level that saturates the elimination processes, in which case first order eliminations processes will change to zero order. 

The time taken for the concentration of a drug to fall to half its original value is known as its biological half-life (/l/2). For eliminations that follow first order kinetics it may be shown that  

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