adverb
no medical treatment having proved effective
A formal, somewhat literary set phrase used when all medical efforts have failed, often in contexts of terminal illness or death.
祖父は長い闘病の末、薬石効無く亡くなった。
After a long illness, my grandfather passed away with no medical treatment having proved effective.
Standard kanji spelling for this set phrase.
Variant spelling using okurigana 効なく instead of 効無く.
A Sino-Japanese compound from 薬石 (medicine and stone, i.e., medical treatment) and 効無く (without effect). The exact historical derivation is uncertain, but it is a fixed literary expression.