Boxed Warning

A boxed warning is the most prominent element of United States prescribing information, printed inside a border at the very top of the label. Regulation reserves it for narrow circumstances: an adverse reaction so serious it must be weighed against benefit before prescribing, a serious risk that can be reduced by patient selection or monitoring, or a drug approved only under restricted distribution. The FDA can require one at approval or add it afterwards, and its presence restricts promotion, since reminder advertising is barred for a product carrying one.

The incretin class is the worked example. Liraglutide, dulaglutide, extended-release exenatide, semaglutide and the GIP and GLP-1 co-agonist tirzepatide all carry a boxed warning for thyroid C-cell tumours, paired with a contraindication in anyone with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2. The warning rests on rodent bioassays, not on human cases. Twice-daily exenatide does not carry it, which shows the warning tracks product-specific data rather than the receptor.

A boxed warning states severity and required prescriber behaviour, not probability. It says nothing about how often the event occurs, and in several instances the underlying evidence is preclinical. Read correctly it tells you which screening questions to ask, which histories exclude a patient, and where the regulator judged ordinary label text insufficient.

Both failure modes appear constantly. One is reading the box as proof of a demonstrated human hazard, when the C-cell warning derives from a rodent finding whose human relevance remains unresolved. The other is reading its absence as reassurance. Compounded, research-grade and grey-market material carries no approved label, so no boxed warning can appear on it; that is a missing document, not a missing risk.