What is a boxed warning? It's the strongest safety warning the FDA can require on a prescription drug's label. It sits in a bordered box at the top of the prescribing information, the long official document written for prescribers, and it flags a risk serious enough that it has to be weighed before the drug is prescribed. Most people call it a "black box warning." Same thing.
What it isn't matters just as much. It isn't a ban, and it isn't a verdict that a drug is unsafe for everyone. Drugs with a box stay on the market and are prescribed every day. The box says where the serious risk sits and, often, who shouldn't take the drug at all.
If you've read a semaglutide or tirzepatide label, you've already met one: a box about thyroid tumors in rodents.
What is a boxed warning, officially?
The rule lives in federal regulations. Title 21, section 201.57 says FDA may require "certain contraindications or serious warnings, particularly those that may lead to death or serious injury," to be presented in a box. A contraindication is a situation in which the drug shouldn't be used at all.
The same rule sets the format: a capitalized heading that includes the word WARNING, a short explanation of the risk, and a pointer to the fuller discussion further down the label. The Highlights, the summary at the front, carry a shorter version of the box, no more than 20 lines.
One sentence in that rule matters most for GLP-1 readers. A boxed warning "ordinarily must be based on clinical data, but serious animal toxicity may also be the basis of a boxed warning in the absence of clinical data." In plain terms, a box can rest on animal findings alone.
Here is what that looks like. The current Zepbound label on DailyMed opens with WARNING: RISK OF THYROID C-CELL TUMORS, then: "In rats, tirzepatide causes dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures. It is unknown whether ZEPBOUND causes thyroid C-cell tumors, including medullary thyroid carcinoma (MTC), in humans…"
Why a drug gets a boxed warning
FDA's 2011 guidance on writing these label sections names three usual reasons:
- a reaction so serious next to the drug's benefit (fatal, life-threatening or permanently disabling) that it has to be part of every risk-benefit decision;
- a serious reaction that careful use can prevent or reduce, for example by choosing patients carefully or monitoring them;
- a drug that could only be approved with limits on how it is distributed or used.
Boxes are "most likely to be based on observed serious adverse reactions," the guidance says, but they can also cover a reaction anticipated from animal studies or from other drugs in the same class. Hold on to that last part.
The GLP-1 boxed warning, explained
The branded GLP-1 weight-loss medicines share one kind of box. It concerns thyroid C cells, the cells that make a hormone called calcitonin. Medullary thyroid carcinoma (MTC) is a rare cancer that starts in those cells, and MEN 2 (multiple endocrine neoplasia type 2) is an inherited condition that raises the risk of it.
As of September 22, 2026, the day this article was written, the current DailyMed labels for Wegovy ("Revised: 6/2026"), Zepbound ("Revised: 8/2026") and the newer Foundayo pill ("Revised: 7/2026") all opened with the thyroid box. The Wegovy and Zepbound boxes say three things: the drug caused thyroid C-cell tumors in rodents; the human relevance of those tumors "has not been determined"; and people with a personal or family history of MTC, or with MEN 2, must not take it.
Why rodents? A 2010 study by Novo Nordisk scientists found GLP-1 receptors on rat and mouse C cells, where GLP-1 drugs triggered calcitonin release and cell growth. Human and monkey C cells had few of those receptors, and 20 months of liraglutide at more than 60 times human exposure didn't cause that growth in monkeys. The authors still wrote that long-term effects on the human thyroid "remain unknown." That open question is why the box stays.
Foundayo (orforglipron), a once-daily GLP-1 pill first approved in 2026, shows the class logic plainly. Its label says orforglipron "is not pharmacologically active in rats or mice and did not produce tumors in rodents," yet it carries the box anyway, because it acts on the same human receptor as the drugs that did.
What the human studies show is a separate question, and two guides own it: tirzepatide and thyroid cancer and whether semaglutide causes cancer.
What a black box warning does and doesn't mean
| What people assume | What the box actually tells you |
|---|---|
| "The drug is about to be pulled" | The drug stays on the market; the box keeps a serious risk impossible to miss |
| "It's unsafe for everyone" | It often names who shouldn't take it; for GLP-1s, people with an MTC or MEN 2 history |
| "It proves harm in people" | The rule allows animal data alone; the GLP-1 box says human relevance has not been determined |
| "No box means no serious risks" | The Warnings and Precautions section lists serious risks too, such as pancreatitis on GLP-1 labels |
The other common mistake is thinking a box is fixed on the day a drug launches. When FDA learns of new safety information, federal law gives the company 30 days to propose a label change, "including changes to boxed warnings," and FDA can ultimately order one.
It happens often. Of 548 new drugs approved from 1975 to 1999, 45 later picked up at least one boxed warning, about 1 in 12 (Lasser et al., JAMA, 2002). A study of 222 new drugs approved from 2001 to 2010 counted 61 boxed warnings added after launch (Downing et al., JAMA, 2017).
Boxes come off, too. On February 12, 2026, the agency approved label changes for six menopausal hormone therapy products that removed statements about cardiovascular disease, breast cancer and probable dementia from their boxed warnings.
Compounded medications and the boxed warning
A compounded medication, one prepared by a licensed pharmacy for an individual patient on a prescription, has no prescribing information of its own, so no box is printed for it. FDA's own Q&A says plainly that compounded drugs are not FDA-approved, which means the agency doesn't verify their safety, effectiveness or quality before they're marketed.
That doesn't make the risk disappear. Compounded tirzepatide is still tirzepatide, so the reviewing provider screens against the branded label for the same molecule, MTC and MEN 2 questions included.
Through Promise, tirzepatide and semaglutide are dispensed as compounded medications, which are different from FDA-approved products: the formulations offered here are not FDA-approved. A licensed provider may still prescribe a compounded formulation; that decision is between you and your doctor. At Promise, a licensed provider reviews every request, and not everyone qualifies.
How to find the boxed warning on a label
The box sits at the start of the full prescribing information, before the Indications section, and a shorter copy appears in the Highlights on page one. The National Library of Medicine's DailyMed site posts current labels free, and each prints its revision date at the end of the Highlights. That date is worth checking, because labels change. Wegovy and Zepbound also come with a Medication Guide, a plain-language handout for patients that covers the same thyroid risk.
Related terms
- Warnings and Precautions: section 5 of the label, one step below the box. In January 2026 FDA asked GLP-1 makers to remove a suicidal-thoughts warning from this section after a review of 91 placebo-controlled trials found no increased risk.
- Other regulatory terms: FDA tentative approval, what an IND is and off-label use.
- Full label walk-throughs: Wegovy prescribing information and Zepbound prescribing information.