Why do some antibiotics need to be taken for several days instead of one dose?

Bacteria multiply at different rates, and antibiotics often work best when blood levels stay above a threshold for a certain period. A single dose may not eliminate all bacteria, especially if the infection is deep, widespread, or only partly sensitive to the drug. Multi-day treatment helps clear remaining bacteria and reduces the chance of relapse. The exact length depends on the infection, the medicine, and the patient’s condition. This is why doctors prescribe specific courses rather than a one-size-fits-all duration.