GLP-1 medications: the risks and questions worth asking first.

GLP-1 receptor agonists — semaglutide (Ozempic, Wegovy) and the dual GIP/GLP-1 agonist tirzepatide (Mounjaro, Zepbound) — have produced the largest weight loss ever seen from medication, and semaglutide has been shown to reduce major cardiovascular events in people with obesity and heart disease.^1,7 None of what follows disputes that. But "does it work" is the question the marketing already answers. The questions that keep patients up at night are about safety — and those deserve an honest, physician-level answer rather than a sales pitch.

The common side effects: your gut

The most frequent side effects are gastrointestinal. Up to 25–40% of patients experience nausea, and roughly 10–20% experience vomiting, diarrhea, or constipation.¹ In most people these are mild to moderate, appear during dose increases, and settle with time. They can often be reduced by eating smaller and slower meals, stopping at the first sign of fullness, and limiting greasy or spicy foods — and by escalating the dose slowly rather than rushing to the top.¹ Persistent vomiting or diarrhea also matters for a less obvious reason: dehydration can injure the kidneys, which is why these symptoms should be reported rather than pushed through.

The serious risks worth knowing

Less common but more serious effects have been reported: gallbladder disease, pancreatitis, and bowel obstruction (ileus). In 2023 the FDA added a warning about ileus to semaglutide's label after receiving reports of intestinal blockage.³ A population-based study found that, compared with an older weight-loss drug, GLP-1 use was associated with higher rates of pancreatitis, bowel obstruction, and gastroparesis (delayed stomach emptying, sometimes called "stomach paralysis" in the press).²

The honest caveat: these were observational findings, and it is not yet clear the drugs directly cause them — the association could partly reflect who takes these medications rather than the medications themselves.² That uncertainty is exactly why an evaluation and ongoing monitoring matter: the point is to catch problems early, not to assume they can't happen.

Who should not take them

These are genuine contraindications, not fine print:³

• Pregnancy — GLP-1 medications should not be used in pregnancy, and are generally stopped a couple of months before trying to conceive. They are also avoided while breastfeeding due to a lack of safety data.
• A personal or family history of medullary thyroid cancer, or MEN 2 (multiple endocrine neoplasia type 2) — an absolute contraindication carried in the boxed warning.
• Caution, or avoidance, with certain conditions — severe inflammatory bowel disease and gastroparesis (because of the risk of severe GI effects), and a history of pancreatitis (generally used only if the original cause has been resolved).
• People with diabetes on insulin or sulfonylureas — not a contraindication, but doses often need lowering to avoid hypoglycemia as weight comes down.

The thyroid cancer question

This is one of the most-searched worries, so it deserves a straight answer. The boxed warning exists because high doses caused thyroid C-cell tumors in rodents. Whether that translates to people has not been established — by the FDA's own labeling, the human relevance of the rodent finding is undetermined. Routine calcitonin testing or thyroid ultrasound screening is of uncertain value and not routinely recommended for people on these medications — but the personal- and family-history contraindication above still stands.³

Muscle loss: the concern the ads skip

When you lose weight quickly, not all of it is fat. In the STEP 1 body-composition analysis, a meaningful share of the weight lost was lean body mass rather than fat — on the order of a third or more.¹ Losing muscle in your 40s and beyond is not a cosmetic footnote; it affects strength, metabolism, and long-term function. It is also largely preventable — which is why adequate protein and resistance training belong alongside the medication from the start, not as an afterthought. A prescription with no plan for muscle is doing half the job.

"Ozempic face," hair loss, and the price of speed

Two things people notice — facial hollowing (so-called "Ozempic face") and temporary hair shedding — are consequences of losing a lot of weight quickly rather than toxic effects of the drug: fat leaves the face along with everywhere else, and rapid weight loss can trigger a self-limited phase of hair shedding. The practical lesson is the same as with muscle: a steadier pace, adequate protein, and attention to nutrition make a real difference.

Surgery, anesthesia, and a newer eye concern

Because these medications slow stomach emptying, food can remain in the stomach longer than expected — which raises the risk of aspiration under anesthesia. Anesthesiology guidance now recommends that GLP-1 medications may need to be held before elective surgery or procedures requiring sedation, so tell any surgeon or anesthesiologist that you take one.^3,5

More recently, regulators have examined a rare eye condition — non-arteritic anterior ischemic optic neuropathy (NAION), a sudden loss of blood flow to the optic nerve. In 2025 the European Medicines Agency recommended listing NAION as a very rare side effect of semaglutide, on the order of up to 1 in 10,000 users.^8,9 It is rare, and the overall picture is still being studied — but sudden vision changes are worth taking seriously and reporting promptly.

What about mood and suicidal thoughts?

After early reports, this concern was formally investigated. A review by the European Medicines Agency did not find evidence of a causal link between GLP-1 medications and suicidal thoughts or self-harm.¹⁰ That is reassuring, not dismissive: anyone with a history of depression starting any weight or metabolic treatment benefits from attention to mood, and symptoms should always be raised with your physician.

Compounded and counterfeit versions

With demand outstripping supply, "compounded" and counterfeit semaglutide have flooded the market — and this is a real safety problem, not a bargain. These products are not reviewed for safety, quality, or effectiveness the way approved medications are, and regulators have linked some compounded versions to dosing errors and hospitalizations, including accidental overdoses many times the intended dose.⁴ A medication worth taking is worth taking in its regulated, verified form.

What happens when you stop

Of all the expectations worth setting, this one matters most. In the STEP 1 trial extension, patients regained about two-thirds of their lost weight within a year of stopping semaglutide, with the cardiometabolic improvements reverting alongside.⁶ That reflects the nature of obesity as a chronic, relapsing condition. These medications manage it while taken, much like blood-pressure medication. The question worth asking is therefore what the long-term plan looks like, and what you are building alongside the medication so the results hold.