Something unpleasant that must be accepted or endured.

Alternates

• Tough pill to swallow
• Hard pill to swallow
• Difficult pill to swallow

Usage Notes

• Bitter pill(s) to swallow is not a set phrase. Only a little more than 40% of the usage at COCA with a form of swallow within 9 words before or after is of the form given.
• Other verbs such as take, down, and digest may replace swallow.
• About one third of the time bitter pill appears without any such verb nearby.
• Other adjectives modify pill about 60% of the time: hard, tough, bad, difficult, even easy.

Sources

Wiktionary