It depends on what's in it. Plain black coffee left out overnight is a taste problem, not a safety one — coffee with milk or creamer is a different story.
Coffee gets lumped in with other overnight food-safety questions, but plain brewed coffee doesn't behave like a perishable food at all — it's mostly water, with no protein or dairy for bacteria to feed on.
What actually determines the answer here is whatever got added to the cup, not the coffee itself.
Coffee left out overnight isn't a food-safety risk on its own. Its acidity and lack of protein make it a poor environment for the bacteria that cause foodborne illness. What you will notice is flavor: coffee left out overnight goes stale, picks up a bitter or flat taste as its oils oxidize, and loses its aroma. Some people notice this shift after just an hour or two; others won't mind it much at all.
Once dairy enters the picture, coffee follows dairy's rules, not coffee's. Milk or cream left unrefrigerated for more than about 2 hours can grow bacteria the same way a glass of milk would, and being mixed into hot coffee doesn't sterilize it or extend that window. Don't drink coffee with dairy that sat out overnight, even if it smells okay at first — the same reasoning that applies to a glass of milk left on the counter applies here.
If you want to save black coffee for iced coffee tomorrow, refrigerate it instead of leaving it on the counter — it keeps the flavor closer to fresh and avoids the staling that happens at room temperature. Coffee with dairy should go in the fridge quickly too, though it's still best used within a day or so, same as milk, and should be checked for smell before drinking even after refrigeration. Iced coffee made ahead of time follows this same split: brew it strong and refrigerate the black coffee base, but only add milk or creamer right before serving. This is also why coffee shop cold brew concentrate, which is usually sold plain, keeps for a week or two in the fridge while a latte made from it should be finished the same day.
Plain coffee left out overnight just tastes worse. Coffee with milk or creamer left out overnight is genuinely unsafe to drink — treat it the same way you'd treat leftover milk.
Black coffee, yes — it's still safe, just stale. Reheating coffee with dairy that sat out overnight doesn't fix the safety issue, since heat may not destroy everything bacteria already produced.
No, not for safety. Refrigerating it does help preserve flavor for a day if you plan to drink it cold later, like iced coffee.
Stale black coffee isn't dangerous, just less pleasant — more bitter and flat-tasting as its oils oxidize. It won't make you sick the way spoiled dairy could.
Liquid dairy or refrigerated creamers shouldn't sit out more than about 2 hours, the same rule as milk. Shelf-stable powdered creamers are unaffected by being left out.