USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Which of the following are examples of zero-order drugs?

1. Lithium

2. Alcohol

3. Phenytoin

4. Salicylates

The correct answer is: Alcohol

Zero-order drugs are characterized by their metabolism, which occurs at a constant rate regardless of the drug concentration in the body. This means that the same amount of the drug is eliminated over a given period, leading to a linear decline in drug levels rather than an exponential decline seen in first-order kinetics. Alcohol is a prime example of a zero-order drug. Its metabolism is largely independent of its concentration when consumed in significant quantities, leading to a fixed amount metabolized per unit of time. This unique characteristic is why individuals can experience varying levels of intoxication and slow elimination depending on their consumption patterns. While lithium, phenytoin, and salicylates can exhibit zero-order kinetics at high doses, they typically display first-order kinetics at therapeutic levels. This means that under most circumstances, their metabolism is dose-dependent and varies with concentration. A key point to remember is that zero-order kinetics can occur in certain situations involving saturable metabolism, but alcohol consistently shows zero-order behavior under normal consumption levels, making it a clear choice for this category.