藥 and 樂 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.藥 /*m-r[e]wk/ 樂 /*[r]ˤawk/, /*[r]ˤawk/, /*[ŋ]ˤrawk-s/, /*[ŋ]ˤrawk/
drugs, pharmaceuticals, medicine; medicine; drug; substance used for a specific purpose (e.g. poisoning, explosion, fermenting); CL:種|种[zhǒng],服[fù],味[wèi]; to poison