Is it safe to eat a sandwich with mayonnaise that was left out overnight?

It is not recommended to eat a sandwich with mayonnaise that has been left out at room temperature for more than 2 hours. This is because mayonnaise is made with eggs, which are a high-risk food for bacterial growth. When left at room temperature, bacteria can multiply quickly, and some can produce toxins that can cause foodborne illness.

While some commercial mayonnaise brands may use pasteurized eggs that are less likely to cause illness, it is still best to err on the side of caution and discard sandwiches with left-out mayonnaise. Additionally, if the sandwich contains other perishable ingredients such as meat or dairy products, it is even more important to discard it.

To ensure food safety, it is recommended to keep perishable foods such as sandwiches with mayonnaise refrigerated at 40°F (4°C) or below. If you are unsure about the safety of a sandwich, it is best to discard it rather than risk getting sick.