Intransitive verb and Godan verb
きく
肺癌の治療に最も良く効く薬を探しています。
I’m looking for the most effective medicine for treating lung cancer.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means effective. The verb vocab version is to be effective (since "to effective" doesn't make sense).
What is particularly effective? It's your key (き). It's effective at opening your house door.