A boxed warning is the most serious warning the FDA can require on a prescription drug label. It sits at the top of the label inside a printed border, which is where the informal name "black box warning" comes from.
What it means
It means the FDA has concluded there is a serious risk that must be communicated prominently, and that a prescriber should weigh it explicitly before writing the prescription. The warning describes a hazard, the circumstances in which it arises, and often what monitoring or precaution reduces it.
What it does not mean
It does not mean the drug is being withdrawn, that it is unsafe for everyone, or that a safer equivalent necessarily exists. Many drugs carrying a boxed warning are the standard treatment for their condition and are prescribed to millions of people. The warning exists so the decision is made with the risk in view; it does not make the decision.
It is also not a ranking. A drug with a boxed warning is not automatically more dangerous than one without. The warning reflects a specific identified risk in specific circumstances, and comparing two drugs on the mere presence of a box tells you very little about either.
Why the wording differs between manufacturers
There is no single official text. The FDA sets what a warning must cover and each manufacturer writes its own label within that, so labels for the same drug carry materially different wordings of the same warning. On several drugs we sampled, a quarter or more of the labels differed in wording.
The hazard being described does not change. The sentences do. This explains something that otherwise looks alarming: a refill dispensed with a different manufacturer's product can arrive with warning text that reads differently from last month's, without anything about the drug or the risk having changed.
Where to read the actual warning
Every drug page in this section quotes one identified label and links it on DailyMed, the National Library of Medicine's index of current FDA labelling, so the exact wording can be read in full rather than summarised. Browse the drugs covered.