Loyal advice grates on the ear, but bitter medicine cures the illness.
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From 孔子家语 (Liuben), and echoed in 史记 (Liuhou Shijia) where Zhang Liang uses it to admonish Liu Bang: 良药苦口利于病，忠言逆耳利于行 - 'good medicine is bitter in the mouth but good for illness; loyal advice grates on the ear but good for conduct.' The full couplet is one of the most quoted maxims in the Confucian tradition.
usage
Nearly always cited together with its twin 忠言逆耳利于行. The shorter idiom 良药苦口 is the canonical chengyu form; the longer version here foregrounds the pragmatic punchline 利于病 ('good for the illness').
