A lot of people panic the moment they see mayonnaise sitting on the counter instead of in the fridge. It feels risky, almost guaranteed to make someone sick. But the truth is more nuanced than most of us were ever told — and that’s why so many grandmothers have done this for decades without issues.

Store-bought mayonnaise is not the same as homemade mayo. Commercial mayonnaise is made with pasteurized eggs and a high level of acidity, usually from vinegar or lemon juice. That acidic environment makes it very difficult for harmful bacteria to grow quickly. Because of this, unopened mayo can sit at room temperature for months, and even after opening, it’s far more stable than people assume.

That said, once opened, mayonnaise should not be left out for days at a time. Food safety experts generally agree that opened store-bought mayonnaise can safely sit out for up to 8 hours at room temperature. After that, the risk of spoilage and bacterial growth increases, especially in warm environments. Leaving it out for “a few days” is not considered safe.

Homemade mayonnaise is a completely different story. Because it’s often made with raw eggs and lacks preservatives, it should never be left out and must be refrigerated immediately. Even a few hours on the counter can make it unsafe.

There are also warning signs people often ignore. If mayonnaise smells sour, looks separated, turns yellow or watery, or tastes “off,” it should be thrown away immediately. When in doubt, it’s not worth the risk.

So why did nana do it? In many older kitchens, mayonnaise was used quickly, kept in cooler homes, and often didn’t sit untouched for long. Times — and room temperatures — have changed.

The bottom line: unopened mayo is shelf-stable, opened mayo should be refrigerated, and leaving it out for days is not safe. Discovering this late is common — but knowing it now can save a lot of stomach pain later.