What are first-order kinetics?

In first-order kinetics a constant proportion of the drug is eliminated from the body per unit time, e.g. 50% per hour and therefore clearance and half-life can be used to describe elimination kinetics.

The rate of elimination varies and is directly proportional to the concentration of drug in the body at that time. This produces an exponential decay curve and is due to non-saturatable enzymes being involved in drug elimination.

The majority of drugs display first-order kinetics because the body contains more enzymes than needed to metabolise the clinically effective dose.

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