Tirzepatide

Is tirzepatide safe?

For most adults with type 2 diabetes or obesity, tirzepatide is a reasonable choice with known trade-offs. It is the active ingredient in Mounjaro and Zepbound, made by Eli Lilly, and works on two gut hormone receptors (GIP and GLP-1) rather than one. That makes it a close relative of semaglutide, with a similar side effect pattern.

Three things decide the score. Stomach side effects are common: in the Zepbound weight trials, 25% to 29% of people had nausea, against 8% on placebo. Serious problems such as pancreatitis and gallbladder disease are uncommon but on the label. And the drug is young, first approved in May 2022. We score it 70, just below semaglutide (72), mainly because it has a shorter record and an extra interaction with the contraceptive pill.

What are the common side effects?

The Zepbound label pools two 72-week trials with 2,519 people on the drug:

Side effect	Tirzepatide 5 to 15 mg	Placebo
Nausea	25% to 29%	8%
Diarrhoea	19% to 23%	8%
Vomiting	8% to 13%	2%
Constipation	11% to 17%	5%
Hair loss	4% to 5%	1%

Most of this happens while the dose is going up. Between 4.8% and 6.7% of people stopped because of side effects, against 3.4% on placebo, mostly in the first months. Hair loss was linked to weight loss and was far more common in women (7.1%) than men.

The serious risks

• Pancreatitis. Uncommon, but cases after approval include haemorrhagic and necrotising pancreatitis, sometimes fatal. In January 2026 the UK regulator strengthened warnings for the whole drug class and reminded doctors that privately prescribed weight-loss injections may not appear in a patient's record.
• Gallbladder disease. Cholecystitis was reported in 0.7% of people on tirzepatide vs 0.2% on placebo.
• Kidney injury. Usually from dehydration after vomiting or diarrhoea; 0.5% vs 0.2% in the weight trials, and some post-marketing cases needed dialysis.
• Aspiration under anaesthesia. Food stays in the stomach longer, so tell any surgery or endoscopy team you take it.
• Low blood sugar. Mainly with insulin or sulfonylureas; in the diabetes weight trial, 4.2% had glucose below 54 mg/dL vs 1.3% on placebo.
• Severe allergic reactions, including anaphylaxis and angioedema.

The boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary covers thyroid C-cell tumours, which tirzepatide caused in rats at exposures similar to human ones. Whether it does this in people is unknown. It is not used by anyone with a personal or family history of medullary thyroid cancer or MEN 2.

Doses on the label

Both brands start at 2.5 mg injected once a week. That first dose is for getting used to the drug, not a treatment dose. It goes up in 2.5 mg steps, at least 4 weeks apart, to a maximum of 15 mg a week. Zepbound's maintenance doses are 5, 10 or 15 mg (10 or 15 mg for sleep apnoea). Mounjaro is also approved for children 10 and older with type 2 diabetes, up to 10 mg a week.

The current Mounjaro label also covers lowering heart risk in type 2 diabetes. In SURPASS-CVOT, 13,299 people with diabetes and heart disease were followed for a median of about four years; tirzepatide was non-inferior to dulaglutide for heart attacks, strokes and heart deaths (hazard ratio 0.92), but not proven better.

The contraceptive pill problem

This is the interaction most people miss. Tirzepatide slows the stomach, which can reduce absorption of oral medicines. The label tells people on the pill to switch to a non-oral method, or add a barrier method, for 4 weeks after starting and for 4 weeks after every dose increase. Patches, rings, implants and coils are not affected. People on warfarin or other narrow-margin drugs need closer checks.

Fake pens and compounded copies

Demand has drawn in unofficial supply. The FDA says it had received more than 730 adverse event reports linked to compounded tirzepatide by 31 May 2026, including products used at doses beyond the approved label. In the UK, the MHRA found falsified Mounjaro KwikPen 15 mg pens (batch D873576) sold by one online pharmacy in February 2026; the dose knob came off during use. Buy only from a licensed pharmacy with a prescription, and check the batch and pen strength match the box.

Who should be careful

Talk to your doctor or pharmacist before starting if you have had pancreatitis, gallbladder disease, diabetic eye disease, kidney problems or severe gastroparesis, or if you are planning a pregnancy. People on insulin or a sulfonylurea may need their doses lowered by their prescriber. Supplements sold for blood sugar, such as berberine, can add to the glucose-lowering effect. For people who need an older, cheaper option first, metformin has a much longer record.

How we scored it

Dependence (95) is strong: no misuse or withdrawal. Interactions (75) lose points for the pill and for low blood sugar with other diabetes drugs. Regulatory warnings (45) reflect the boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary, the 2026 pancreatitis update and fake and compounded products. Track record (60) is the weakest point after only four years on sale, and serious side effects (65) carries the most weight. A lower score does not mean you should stop; talk to your prescriber first.