Chill Pill

When we tell someone to take a chill pill means we telling them a calm down and relax.

Examples:
1. She was getting so stressed about what her boss said to her that I told her to just take a chill pill - everything would be fine.
2. My wife's working so hard, she need to take a chill pill, otherwise she'll make herself ill.

Source: http://www.bbc.co.uk