【give it a second thought】 —reassess an opinion or decision previously made:

• You may think this will happen in the distant future but if you 【give it a second thought】 you’ll realize it is already happening.

Note: The expression is not antonymous in meaning to the phrase don’t 【give it a second thought】 —(also: don’t give it a thought)

1. dismiss it from your mind; let that not worry you:

• Don’t 【give it a second thought】 , until you are a lot older and have more experience.

2. is used to indicate that thanks are unnecessary:

• Oh, don’t 【give it a second thought】 , Hun! Actually I should be thanking you!

3. is used as a polite way of accepting apolog y:

• I looked at her and said graciously: “Please don’t 【give it a second thought】 . I wasn’t offended.”

【have second thoughts】 —(also: get second thoughts) realize that one may be mistaken; have doubts:

• I am starting to 【have second thoughts】 about my purchase based on what I am reading here.

Note: The expression is not equivalent in meaning to the phrase have another think coming—used of a need to reconsider one’s plans or opinions:

• Those who think Carter is still respectable company should have another think coming.

See also: think again / think twice.