良药苦口 (liáng yào kǔ kǒu): good medicine tastes bitter ; fig. frank criticism is hard to swallow

good medicine tastes bitter ; fig. frank criticism is hard to swallow

Good medicine is bitter in the mouth. Said just before handing over criticism, or afterwards by the person who has decided to swallow it.

The same saying in Japanese: 良薬は口に苦し

A Chinese idiom (成语)