Oral drugs are often very potent in order to achieve relatively longer efficacy. Medicine for treating colds tends to make people sleepy, but the effect does not remain at a constant level. After several hours the drug wears off and it is necessary to take another dose.
此外，口服藥劑為了達到較長藥效，藥性通常較強，像感冒藥就常讓人昏沉欲睡，但是其藥效不能維持固定，幾個小時後藥力又降低，所以要再服用。