A boxed warning, informally called a black box warning, is the most serious warning the FDA can require. It appears at the very top of prescribing information, surrounded by a border, and highlights risks that can lead to death or serious injury.
The presence or absence of a boxed warning is a useful piece of context when comparing medicines, but it is not a complete measure of risk. A medicine without one can still cause harm, and a medicine with one may still be the right choice for a particular person.