Elimination rate is a constant quantity of drug per unit of time. The amount of drug eliminated is not proportional to the drug plasma concentration.
System can become saturated. Drug plasma concentration can increase disproportionately to dose.
It can be hard to predict how to adjust a drug regimen to obtain a required drug concentration.
Only a few drugs have zero order elimination.
FIRST ORDER ELIMINATION
Elimination rate is a constant proportion of drug per unit time. The amount of drug eliminated is directly proportional to the drug plasma concentration.
System adapts to changes in plasma drug concentration; it cannot become saturated. Drug plasma concentration changes proportionately to dose.
It is easier to predict how to change a drug regimen to obtain a required drug concentration.
In therapeutic ranges, most (about 95 %) drugs have first order elimination.