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As previously mentioned, most drugs display first‐order kinetics where the rate of elimination is proportional to the amount of drug in the body. However, drugs such as ethanol, phenytoin and heparin have zero‐order kinetics. Here, a constant amount of drug is eliminated per unit time, independent of plasma drug concentration. This occurs when the enzymes responsible for metabolism become saturated and the rate of elimination does not increase in response to an increase in concentration (or an increase in the amount of drug in the body) but becomes constant.

The clinical relevance of non‐linear kinetics is that a small increase in dose can lead to a large increase in concentration. This is particularly important when toxic side effects are closely related to concentration, as with phenytoin.