Who should not take a GLP-1

Most people who are offered a GLP-1 can take one. But the exclusions in this class are unusually specific, several of them are things people do not know about themselves, and one of them turns on a family history that is rarely discussed at a routine appointment.

This page sets out what appears on the labels and why. It is written so you know which questions to raise — not so you can decide the answer.

The absolute exclusions

Medullary thyroid carcinoma and MEN 2

Every injectable product in this class carries a boxed warning about thyroid C-cell tumours. In rodent studies, GLP-1 receptor agonists caused these tumours at clinically relevant exposures. Whether the same applies to humans has not been determined — rodents have a far higher density of the relevant receptors in thyroid tissue — but the warning is unambiguous in its practical consequence.

A personal or family history of medullary thyroid carcinoma, or of Multiple Endocrine Neoplasia syndrome type 2, rules out these medicines. This is one of the few exclusions in medicine that depends on what happened to a relative, which means it is easy to miss — many people have never asked what a grandparent's thyroid cancer actually was.

Pregnancy and planned pregnancy

The weight-management products are not for use in pregnancy. Weight loss during pregnancy offers no benefit to a developing fetus, and animal studies raised concerns that have not been resolved in humans.

Because these medicines clear slowly, stopping is not instant. Guidance generally calls for discontinuation a substantial period before a planned conception — the interval differs by product and is a question for your prescriber.

There is a further complication that catches people out: delayed gastric emptying can affect the absorption of oral contraceptives, particularly during escalation. This deserves its own conversation, and we cover it here.

Previous serious reaction

A previous severe hypersensitivity reaction to the product or any of its components is an exclusion, as it would be with any medicine.

Conditions that require caution, not exclusion

These are the ones where the answer is a conversation. Being in one of these groups does not mean you cannot take a GLP-1; it means the decision is individual and the monitoring is different.

Previous pancreatitis

Pancreatitis appears in the warnings for every product in this class, although a causal relationship has not been established. A previous episode is not an automatic exclusion — but it changes the risk calculation and the threshold for investigating abdominal pain later. The symptoms that matter are here.

Gallbladder disease

Rapid weight loss of any cause increases gallstone formation, and this class produces rapid weight loss. Gallbladder events appear in the trial data. A history of gallstones is worth stating explicitly.

Severe gastrointestinal disease and gastroparesis

These medicines work partly by slowing the stomach. In someone whose stomach is already emptying abnormally slowly — diabetic gastroparesis, for instance — that is an obvious problem, and severe gastrointestinal disease generally calls for caution.

Diabetic retinopathy

Rapid improvement in blood glucose can transiently worsen existing diabetic retinopathy. This is a known phenomenon with any rapid glucose correction, not unique to this class, but it means existing eye disease needs monitoring rather than assuming improvement is uniformly good.

Kidney disease

The medicines themselves are not directly nephrotoxic. The risk is indirect and real: severe vomiting or diarrhoea causing dehydration, and dehydration causing acute kidney injury. Existing kidney impairment lowers the margin.

Type 1 diabetes

These products are not licensed for type 1 diabetes. They are sometimes used off-label in specific circumstances, which is a specialist decision and not a general one.

Medicines that need rethinking

A slower stomach changes how other oral medicines are absorbed. Most of the time this does not matter. It matters for medicines where the margin between an effective and a harmful dose is narrow.

• Insulin and sulfonylureas. The combination raises hypoglycaemia risk, and doses are often reduced when a GLP-1 is started.
• Warfarin and other anticoagulants. Changed absorption can shift control; monitoring may need to be more frequent at first.
• Thyroid hormone. Narrow margin, absorption-dependent.
• Oral contraceptives. Particularly during escalation and in the presence of vomiting.
• Other GLP-1 products or DPP-4 inhibitors. Not taken together — the mechanisms overlap.

The practical step is simple and frequently skipped: give your pharmacist a complete list of everything you take, including things you would not think of as medicines. They are better placed than anyone to spot the interaction that matters. We cover this in more detail here.

Age, and the people who are not in the trials

Licensing differs by product and region for adolescents, and several products now have paediatric indications where they previously did not. In older adults the medicines are used, but the risks that matter shift: dehydration is less well tolerated, sarcopenia is a larger concern, and the loss of lean mass that matters for everyone matters more when there is less of it to start with.

What to tell your clinician before you start

Five things, of which the first two are the ones most often omitted:

1. Any family history of thyroid cancer — and which type, if you know.
2. Whether you are pregnant, might be, or are planning to be.
3. Any previous pancreatitis, gallbladder disease or gastrointestinal surgery.
4. Every medicine and supplement you take.
5. Any history of an eating disorder — this class interacts with disordered eating in ways that need supervision rather than avoidance.

None of these is a reason to expect a refusal. All of them are reasons the conversation should happen before the first injection rather than after a problem.

Related

• Pancreatitis: what to watch for — Rare, serious, and easy to dismiss when nausea and abdominal discomfort have become normal. How to tell the difference.
• Drug interactions on a GLP-1 — A slower stomach changes how every oral medicine is absorbed. For most that is irrelevant — for a handful it is not, and those are worth knowing by name.
• Surgery, endoscopy and anaesthesia — A stomach that empties slowly may still hold food after a standard fast. This is the interaction people forget, and the one with the sharpest consequences.
• Pregnancy, contraception and fertility — Weight-management GLP-1s are not for use in pregnancy — and a slower stomach can affect the pill. Two facts that together catch people out.

Which medicine you are on

Everything above applies across the class, because it comes from a shared mechanism. The numbers that differ — dose ladders, missed-dose windows, in-use storage periods, half-lives — differ by product, sometimes substantially. Each medicine has its own reference here:

Ozempic · Wegovy · Mounjaro · Zepbound · Rybelsus · Saxenda · Victoza · Trulicity · all medicines