Efficacy and efficiency are related, but they answer different questions.
Efficacy asks, Does it produce the intended result? If a medicine lowers blood pressure as intended, it has efficacy. This word focuses on success or effectiveness.
Efficiency asks, How well are resources used? An efficient process saves time, effort, money, or materials while still doing the job. This word focuses on minimal waste.
The key contrast is simple: something can work well without working smoothly. A team might reach its goal, which shows efficacy, but use too many meetings and too much time, which means it is not efficient.
- The treatment was effective in reducing symptoms.
- The clinic became more efficient after it simplified paperwork.
- A method can be effective but inefficient.
Use efficacy when you mean proven ability to produce a result, especially in technical, medical, or academic contexts. Use efficiency when you mean doing something with less waste.