Guide

GLP-1 Side Effects: What the Trials Recorded, Effect by Effect

At tirzepatide 15 mg in SURMOUNT-1: nausea 29%, diarrhoea 23%, constipation 17%, vomiting 13%, dyspepsia 10%. Most were mild to moderate and clustered around dose increas

What was actually recorded

The distribution matters more than the headline. Events clustered during escalation rather than spreading evenly through the 72 weeks, which is why the first twenty weeks are the hard part and why the label directs increases no sooner than every four weeks, based on tolerability.

Effect by effect

• Nausea — the most common reason people consider stopping, and the one where holding a dose is most often the right answer.
• Constipation — and the line between slowed transit and ileus, which the label flags and which presents very differently.
• Fatigue — three causes worth separating, because they point to different answers.
• Hair loss — almost certainly telogen effluvium from the rate of weight loss rather than a drug effect, which changes what you can do about it.

Effects on specific organs and systems

• Kidneys — demonstrated benefit over years, and acute injury reported through dehydration over days. Both are true.
• Gallbladder — rapid weight loss is the risk factor, and the symptoms that mean same-day rather than next-appointment.
• Vision — two separate issues that get merged in almost every account of them.
• Bone — what rapid weight loss does to a skeleton, and the fracture question the trials have not run long enough to answer.
• Gastroparesis — where the intended mechanism ends and a diagnosis begins.
• Mental health — what the EMA and FDA reviews actually concluded.

The pancreatitis signal, quantified

On 29 January 2026 the UK medicines regulator updated GLP-1 product information and issued a safety alert on acute pancreatitis. Between January 2023 and June 2026 there were 1,176 reported cases of acute and chronic pancreatitis following tirzepatide use, 18 of them fatal.

Read honestly: spontaneous reporting captures a fraction of events and does not establish causation, and this sits against many millions of patient-months. What it establishes is why the label's symptom description is specific. Severe, persistent abdominal pain radiating to the back is not something to wait out.

Expected, contact, urgent

Expected: nausea in the days after a dose increase, reduced appetite, early fullness, mild fatigue during escalation, bowel changes in either direction.

Contact your prescriber: symptoms that do not settle between doses, vomiting that limits fluids, dizziness on standing, weight falling faster than intended, or anything that makes you consider stopping.

Same day: severe abdominal pain particularly radiating to the back; persistent vomiting with signs of dehydration; severe right-upper pain with fever or jaundice; abdominal distension with inability to pass stool or gas; sudden vision loss in one eye.

Questions readers actually ask