If something is efficacious, it works in the way you intended: it has the power to produce the right result. Doctors, scientists, and rather formal writers like this word — an efficacious medicine, an efficacious method, an efficacious treatment. It does not mean the thing is quick, cheap, or neat. It means it actually does the job. In ordinary talk, people just say it works, or it is effective. Efficacious sounds like a paper or a textbook. Though actually, students often reach for efficient by mistake. Efficient is about little waste: a fast route, a clever use of time. A medicine can be slow and still be efficacious; a machine can be efficient and still do the wrong thing. In Taiwan you may also hear a warmer everyday cousin for remedies, 靈驗, when something really helps — but efficacious itself stays in the formal English of tests and research.