Is it safe to drink a banana smoothie that's been left overnight in the fridge?

Yes, it is generally safe to drink a banana smoothie that has been left overnight in the fridge, provided it was stored in an airtight container and the refrigerator is functioning properly. However, the texture and taste may change due to separation or browning of the banana. Always check for any off smells or unusual appearance before consuming. If in doubt, it's best to err on the side of caution and discard it.