GLP-1 side effects, with the trial numbers

The GLP-1 drugs slow the stomach, act on appetite in the brain, and change how the pancreas responds to food. Almost every common side effect follows from the first of those. The rare ones are on the label as warnings, and the label is worth reading once.

The common ones

From the pivotal trials, at full dose over more than a year. The placebo rates are in brackets, because some of this happens on nothing.

Also common and less talked about: belching, reflux and heartburn, bloating, fatigue in the first weeks, headache, dizziness, hair shedding a few months in (usually from the speed of weight loss rather than the drug), a small rise in resting heart rate, and injection site reactions. Most peak in the first two weeks of a dose step and fade by week four.

The ones on the label as warnings

• Thyroid C-cell tumours. Seen in rodents, not shown in people, but every GLP-1 label carries the boxed warning. Do not take these drugs with a personal or family history of medullary thyroid carcinoma or MEN 2.
• Pancreatitis. Severe, persistent pain in the upper abdomen, sometimes through to the back, with or without vomiting. Stop and call the same day.
• Gallbladder disease. Gallstones and cholecystitis are more common, partly from rapid weight loss. Pain under the right ribs after eating, fever, or yellowing skin are the signs.
• Low blood sugar. Mainly when combined with insulin or a sulfonylurea. On a GLP-1 alone it is rare.
• Kidney injury, almost always from dehydration after prolonged vomiting or diarrhoea. Fluids matter.
• Severe gut problems. Ileus (the gut stopping) is on the labels. Severe constipation with abdominal swelling and vomiting is a call, not a note.
• Aspiration under anaesthesia. Added to the labels in 2024. Tell any surgical team you are on a GLP-1 and when the last dose was.
• Mood. The weight-loss labels used to carry a warning about suicidal thoughts. In January 2026 the FDA asked for it to be removed after a review of 91 placebo-controlled trials with 107,910 patients found no increased risk. A change in mood is still worth reporting, whatever the cause.
• Allergic reactions. Rare. Swelling of the face or throat, trouble breathing, a widespread rash.
• Diabetic retinopathy. In people with existing eye disease, rapid improvement in blood sugar can temporarily worsen it. Keep the eye appointments.

What to call about today

Severe or persistent abdominal pain. Vomiting that will not stop, or that stops you keeping fluids down. Signs of an allergic reaction. Yellow skin or eyes. A fast heartbeat that does not settle. Thoughts of harming yourself. None of these wait for the four-week review.

Tracking it

Everything in the first table has a shape over the week after the shot and over the four weeks of a dose step. A log that captures the day after the dose, the severity and the step is what turns "I feel sick a lot" into a decision. The side effects tracker page explains what to write down, and the week by week page shows the usual curve.