Efficacy asks does it work, as in a drug's efficacy against a virus. Efficiency asks does it work well, without wasting time or resources. A treatment can have high efficacy but be delivered inefficiently, so the two aren't interchangeable.
The core difference
Efficacy measures whether something produces the desired result; efficiency measures how well resources are used in producing it.
- efficacy — the power to produce a desired result: Clinical trials confirmed the drug's efficacy against the virus.
- efficiency — using time, energy, or resources well without waste: The new software improved the factory's efficiency significantly.
How to tell them apart
Efficacy is most at home in clinical and scientific contexts, where it describes whether a treatment or method actually produces the intended effect under controlled conditions. Efficiency belongs wherever resources are involved, whether that's a factory, a schedule, or an engine, and it's about getting a result without unnecessary waste.
A quick test: if you're asking whether something works at all, reach for efficacy. If you're asking whether it works without wasting time, money, or effort, reach for efficiency. Note also that efficiency has a second, unrelated sense: a small self-contained apartment with one main room, as in they rented an efficiency near campus.
