苦口良药 bitter medicine
Explanation
比喻忠告或批评虽然使人一时不快，但对改正缺点和错误很有好处。
A metaphor for sincere advice or criticism, although unpleasant at first, that helps to correct shortcomings and mistakes.
Origin Story
从前，有一个年老的医生，医术高明，救人无数。但他开的药方总是苦不堪言，很多人都不愿意吃。一天，一个年轻的医生问他：“师父，您的药方为什么总是那么苦？病人都不愿意吃。”老医生微微一笑，说：“良药苦口利于病，忠言逆耳利于行。这世间哪有包治百病的仙丹妙药，又哪有只说好话不批评的良师益友？苦口良药，才能治好顽疾；忠言逆耳，才能使人进步。那些只顾眼前的舒服，而不顾长远利益的人，是不会有真进步的。”年轻医生听后，似有所悟。
Once upon a time, there was an old doctor who was known for his excellent medical skills and countless lives saved. However, his prescriptions were always extremely bitter, and many patients were unwilling to take them. One day, a young doctor asked him, “Master, why are your prescriptions always so bitter? Patients don’t want to take them.” The old doctor smiled slightly and said, “Good medicine tastes bitter, but it cures the disease; honest words may sound unpleasant, but they are useful for progress. There is no miracle cure-all medicine, and there is no good teacher or friend who only speaks nice words and offers no criticism. Bitter medicine can cure serious illnesses; sincere words, even if unpleasant, lead to progress. Those who only care about present comfort but not long-term benefits will never truly progress.” The young doctor listened attentively and seemed to have understood something.
Usage
用于比喻忠告或批评虽然使人暂时不愉快，但对自身有益。
Used to describe sincere advice or criticism that, although unpleasant at first, is ultimately beneficial.
Examples
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老师的批评虽然很严厉，但这确实是苦口良药。
lǎoshī de pīpíng suīrán hěn yánlì, dàn zhè quèshì kǔkǒu liángyào.
Although the teacher's criticism was severe, it was indeed a bitter pill to swallow.
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良药苦口利于病，忠言逆耳利于行，这句老话很有道理。
liángyào kǔkǒu lì yú bìng, zhōngyán nì'ěr lì yú xíng, zhè jù lǎohuà hěn yǒu dàolǐ
Good medicine tastes bitter but is good for the disease, honest words may sound unpleasant, but are useful for progress. This old saying makes much sense.