The half-life of this painkiller is about 4 hours, meaning its effectiveness reduces by half after that time.

half-life 是由 half（一半）和 life（生命、寿命）组成的复合词，字面意思是"半寿命"。 在医学和化学领域，half-life 特指药物或放射性物质的"半衰期"，即某种物质的浓度或活性减少到初始值一半所需的时间。