"효험 vs 효능" They both mean efficacy or effectiveness (typically of a medicine) but have difference connotations. 효험 is used more for something esoteric and revered, like traditional folk medicine and treatments whose workings are not widely understood (험 = experience). 효능 is a modern term commonly used of any medicine, or any agent that produces an effect. It focuses on the ability or potency aspect of it (능 = ability, power). There are also 효력 and 효과. - 효력 (력 = strength): (lasting) effect of a medicine; or other effect such as of a law, policy, etc. - 효과 (과 = result): (detectable) effect of a medicine; or any effect (특수 효과(film), 음향 효과, 선전 효과) "약수를 마시면 효험이 없다" (a little unnatural sounding sentence) => 약수를 마셔도 효험이 없다 / 약수를 마셔 봐야[봤자] 효험이 없다. (Even if you drink spring water, no good comes of it)

효험 means "intended effect" 효능 means "efficiency" 약수를 마시면 효험이 없다 There is no intended effect in drinking mineral water. 비타민의 효능이 없다 The efficiency of vitamin is not exist.