Tirzepatide (Mounjaro, Zepbound): Independent Evidence & Risks

Table of contents

• Evidence summary
• What tirzepatide is
• How it works
• What it is prescribed for
• What works and what does not
• Benefits by claim
• Risks and all side effects
• All interactions
• Who should avoid tirzepatide
• Dosage and how to take it
• Follow the money: who makes it and who funded the evidence
• Related research
• Frequently asked questions
• Sources and funding notes

Evidence summary

Independent evidence and credibility scorecard

Independence tiers: 1 = regulator or government body; 2 = academic team or professional publisher with no declared commercial funding for the work; 3 = independent authors with declared ties to a competitor or the maker; 4 = manufacturer-funded or manufacturer-authored. Credibility runs from A (highest) to D.

Country clustering: our sources lean towards the UK and US, which is where the drug's biggest markets and regulators sit. The independent sources (Chinese, Greek, German-American and British teams) agree with the direction of the Lilly trials on weight and blood sugar. The strongest signal is that a review whose authors have ties to Lilly's main competitor still ranked tirzepatide ahead.

What tirzepatide is

Tirzepatide is a once-weekly injection. It is a lab-made peptide, built on the sequence of the gut hormone GIP. A fatty-acid chain lets it bind to albumin in the blood, which gives it a half-life of about 5 days (FDA label).

• Mounjaro: first approved by the FDA for type 2 diabetes on 13 May 2022 (Drugs@FDA data). In the EU it was authorised on 15 September 2022. The EU licence now covers type 2 diabetes in adults and in children aged 10 and over, plus weight management in adults (EMA). In the UK, Mounjaro is the brand for both diabetes and weight management and comes as a KwikPen holding 4 weekly doses (UK SmPC).
• Zepbound: the US brand for weight management, approved on 8 November 2023 (FDA), and for moderate to severe obstructive sleep apnoea with obesity on 20 December 2024 (FDA). Lilly says Zepbound is the obesity brand in Canada, Japan and the US (Lilly 10-K).
• Legal status: prescription-only everywhere we checked. The MHRA says products sold as powder in vials to be mixed before injection "are not authorised" in the UK (MHRA). In the US, compounded copies were allowed only during an official shortage, which the FDA declared over on 19 December 2024 (FDA order).

How it works

Tirzepatide switches on two receptors: GLP-1 and GIP. Both hormones are normally released by the gut after a meal. GLP-1 is a known regulator of appetite and food intake. The FDA label says only that "nonclinical studies suggest" GIP may add to the effect on food intake, so the extra role of GIP in humans is not fully proven (FDA label).

• Proven in humans: it lowers blood sugar, reduces appetite and body weight, and slows stomach emptying. Slower stomach emptying explains the nausea, the aspiration risk under anaesthesia and the effect on the pill (MHRA 2025).
• Animal data only: animal studies show tirzepatide reaches and activates appetite centres in the brain. In rats, tirzepatide caused thyroid C-cell tumours; whether this applies to people "has not been determined" (FDA label).

What it is prescribed for

Licensed uses

• Type 2 diabetes, with diet and exercise, in adults (UK and EU also from age 10) (UK SmPC).
• Weight management in adults with a BMI of 30 or more, or 27 to 30 with at least one weight-related condition (NICE TA1026, section 2).
• Obstructive sleep apnoea with obesity (US) (FDA label). The EMA product page also refers to HFpEF data in the label (EMA).

Where NICE places it and the NHS rollout

For obesity, NICE recommends tirzepatide only for adults with a BMI of at least 35 and at least one weight-related condition. The BMI cut-off is usually 2.5 lower for people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds. If less than 5% of weight is lost after 6 months on the highest tolerated dose, the clinician should decide whether to continue. NICE rejected Lilly's proposal to cover adults with a BMI of 30 or more and at least one weight-related condition, because the cost-effectiveness estimates were too high (NICE TA1026).

NICE estimates 3.4 million people in England are eligible. NHS England asked for, and got, a "funding variation": up to 12 years to roll it out, with at least 220,000 people treated in the first 3 years, prioritised by need (NICE TA1026, section 4). According to NICE's summary of NHS England's interim guidance, the first group is people with a BMI of 40 or more and at least 4 of these conditions: high blood pressure, abnormal blood fats, sleep apnoea, heart disease, type 2 diabetes. People with a BMI of 35 to 39.9 and 4 conditions follow from around June 2026, then BMI 40 or more with 3 conditions from around March 2027. We could not confirm whether the June 2026 stage started on time (NICE).

For type 2 diabetes, NICE TA924 recommends it only when triple therapy with metformin and two other oral drugs has failed or is unsuitable, and BMI is at least 35, or lower if insulin would affect someone's job or weight loss would help other obesity-related problems (NICE TA924).

What works and what does not

Benefits by claim

Weight loss in obesity

SURMOUNT-1 randomised 2,539 adults without diabetes (average weight 104.8 kg, average BMI 38) to tirzepatide 5, 10 or 15 mg or placebo for 72 weeks. Average weight change was −15.0%, −19.5% and −20.9%, against −3.1% on placebo. At least 5% of body weight was lost by 85% to 91% on tirzepatide, compared with 35% on placebo. A loss of 20% or more was reached by 57% on 15 mg and 3% on placebo (Jastreboff 2022). Results vary widely between people.

The trial was "Supported by Eli Lilly", and five of the eleven named authors were Lilly employees (same source). An independent network meta-analysis of 27 trials also ranked tirzepatide 15 mg among the three most effective treatments, behind only the experimental drug retatrutide (Xie 2024; the authors declared no competing interests, and we could not view its funding statement).

Tirzepatide versus semaglutide (Wegovy, Ozempic)

SURMOUNT-5 compared the highest tolerated doses of each drug in 751 adults with obesity but no diabetes. At 72 weeks, weight fell 20.2% on tirzepatide and 13.7% on semaglutide; waist size fell 18.4 cm versus 13.0 cm (Aronne 2025). Two cautions: the trial was open-label, so everyone knew which drug they took, and it was "Funded by Eli Lilly".

In type 2 diabetes, SURPASS-2 found all three tirzepatide doses lowered HbA1c more than semaglutide 1 mg, by 0.15 to 0.45 percentage points, with 1.9 to 5.5 kg more weight lost (Frías 2021; Lilly-funded). The independent Greek review then compared tirzepatide with the higher 2 mg semaglutide dose that SURPASS-2 did not test. It found HbA1c effects "comparable" with semaglutide 2 mg, but tirzepatide still produced more weight loss at each dose step (Karagiannis 2024).

Preventing type 2 diabetes

In the 1,032 SURMOUNT-1 participants who also had prediabetes, 3 years on tirzepatide cut the rate of new type 2 diabetes from 13.3% to 1.3% (hazard ratio 0.07). After 17 weeks off treatment, the figures were 2.4% versus 13.7%, so some of the effect faded once people stopped (Jastreboff 2025; Lilly-funded).

Heart attack and stroke

SURPASS-CVOT compared tirzepatide with dulaglutide (Trulicity, another Lilly drug with proven heart benefit) in 13,165 people with type 2 diabetes and heart or artery disease. Major heart events occurred in 12.2% versus 13.1% (hazard ratio 0.92, 95.3% CI 0.83 to 1.01). This met the test for "no worse" but missed the test for "better" (Nicholls 2025; Lilly-funded). A publicly funded US study of insurance records found tirzepatide and semaglutide gave similar protection (hazard ratio 1.06, 0.95 to 1.18) (Krüger 2026). Here a Lilly trial and an NIH-funded study point the same way: real heart protection, but no clear edge over semaglutide.

Heart failure with preserved ejection fraction

SUMMIT enrolled 731 people with HFpEF and a BMI of 30 or more. Over a median of 104 weeks, cardiovascular death or worsening heart failure occurred in 9.9% on tirzepatide and 15.3% on placebo (hazard ratio 0.62). Quality-of-life scores improved by 6.9 more points on a 100-point scale. The benefit came from fewer heart-failure events; cardiovascular deaths were 8 versus 5, too few to draw conclusions. Side effects caused 6.3% to stop, versus 1.4% on placebo (Packer 2025; Lilly-funded).

Obstructive sleep apnoea

In two SURMOUNT-OSA trials, people started with around 50 breathing pauses per hour of sleep. At 52 weeks, tirzepatide reduced this by 20.0 and 23.8 more events per hour than placebo, in people without and with CPAP machines respectively (Malhotra 2024; Lilly-funded). The FDA called this the first drug treatment for sleep apnoea (FDA).

Keeping weight off

SURMOUNT-4 gave 670 people tirzepatide for 36 weeks (average loss 20.9%), then randomly switched half to placebo. Over the next year the placebo group regained 14.0% of body weight, while the tirzepatide group lost a further 5.5% (Aronne 2024; Lilly-funded). The publicly funded Oxford review found the same pattern. Across 37 studies of weight-loss medicines, cardiometabolic markers such as blood pressure, blood sugar and cholesterol were projected to return to baseline within about 1.4 years of stopping. In the subgroup of 10 study arms (1,776 people) on semaglutide or tirzepatide, weight came back faster, at about 0.8 kg a month; the review had too little data to analyse heart and blood-sugar markers for this subgroup (West 2026). The maker's trial and an independent review agree: this is a long-term treatment, not a course.

Risks and all side effects

Common side effects (weight-management trials)

Figures are for placebo / 5 mg / 10 mg / 15 mg, pooled from SURMOUNT-1 and SURMOUNT-2 (FDA label, Table 1):

• Nausea 8% / 25% / 29% / 28%
• Diarrhoea 8% / 19% / 21% / 23%
• Vomiting 2% / 8% / 11% / 13%
• Constipation 5% / 17% / 14% / 11%
• Abdominal pain 5% / 9% / 9% / 10%; indigestion 4% / 9% / 9% / 10%
• Injection-site reactions 2% / 6% / 8% / 8%; fatigue 3% / 5% / 6% / 7%
• Hair loss 1% / 5% / 4% / 5% (7.1% of women versus 0.5% of men; linked to weight loss)
• Burping, reflux, wind, bloating and dizziness, each around 3% to 5%
• Heart rate up by 1 to 3 beats per minute on average

Between 4.8% and 6.7% stopped tirzepatide because of side effects, compared with 3.4% on placebo, mostly in the first months (same source). Side effects are worst during dose increases.

Low blood sugar

On its own, tirzepatide rarely causes low blood sugar. The risk rises with insulin or sulfonylureas. In SURMOUNT-2 (people with diabetes), levels below 54 mg/dL occurred in 4.2% versus 1.3% on placebo (FDA label).

Mood and suicidal thoughts

In 2024 the MHRA concluded that the evidence did not support a causal link between GLP-1 drugs and suicidal thoughts, self-harm or depression. That review named five older GLP-1 drugs and did not list tirzepatide (MHRA 2024). Its current guidance applies that conclusion to GLP-1 medicines generally (MHRA). The US Zepbound label lists its suicidal behaviour warning as "Removed" in February 2026 (FDA label). Anyone with low mood should still tell their prescriber.

Overdose and fake products

Because the half-life is about 5 days, an overdose may need days of observation (FDA label). The FDA has received reports of adverse events, some needing hospital care, possibly linked to dosing errors with compounded semaglutide, and further reports linked to compounded semaglutide or tirzepatide prescribed above label doses (FDA). The MHRA warns that illegal products "may be fake, contaminated, incorrectly dosed" (MHRA, December 2025).

All interactions

• Oral contraceptives: in Lilly's study, one 5 mg dose cut the peak level of ethinyl estradiol by 59% and total exposure by 20%, with similar falls for the progestogen. The UK SmPC calls this not clinically relevant in women of normal weight, but says data in women with obesity are limited, so a non-oral method or added barrier method is advised for 4 weeks after starting and after each dose increase (UK SmPC). The MHRA says this applies only to Mounjaro among GLP-1 medicines (MHRA). Implants, coils and injections are not affected (FDA label).
• Insulin and sulfonylureas (gliclazide, glimepiride): higher risk of low blood sugar; doses may need lowering (same source).
• Medicines with a narrow safety margin, such as warfarin: slower stomach emptying can change absorption, so monitoring is advised (same source).
• Other GLP-1 medicines (semaglutide, liraglutide, dulaglutide): switching between them, or between brands, should only be done with medical advice (MHRA).
• Anaesthesia and sedation: see aspiration risk above.
• Liver enzymes: lab studies show low potential to affect CYP enzymes or drug transporters (FDA label).

Who should avoid tirzepatide

Contraindications

• A personal or family history of medullary thyroid cancer, or multiple endocrine neoplasia type 2 (MEN 2) (US label) (FDA label).
• Previous serious allergy to tirzepatide (same source).
• Pancreatitis confirmed while taking it: not a formal contraindication, but the UK SmPC says tirzepatide should not be restarted (UK SmPC).

Use with care

• A past history of pancreatitis: not studied (same source).
• Severe stomach or bowel disease, including severe gastroparesis: not studied (same source).
• Older people and anyone at risk of dehydration (same source).
• Type 2 diabetes with diabetic retinopathy (FDA label).

Pregnancy, planning pregnancy and breastfeeding

Tirzepatide should not be used in pregnancy, and animal studies showed reproductive harm. It should be stopped at least 1 month before a planned pregnancy because of its long half-life (UK SmPC). On breastfeeding, official sources differ. The UK SmPC says tirzepatide "could be considered" in breastfeeding, because only undetectable to very low levels reached milk in 11 women (same source). The MHRA's patient guidance says GLP-1 medicines should not be taken while breastfeeding, because of limited safety data (MHRA). We report both; this decision belongs with a clinician.

Children and adolescents

In the UK and EU it is licensed for type 2 diabetes from age 10, with a maximum of 10 mg a week (UK SmPC). It is not licensed for weight management in under-18s. The US label says safety and effectiveness in children have not been established (FDA label).

Kidney and liver disease

No dose change is needed for kidney or liver impairment, but kidney function should be checked if vomiting or diarrhoea could cause dehydration (same source).

Dosage and how to take it

These are label doses, reported for information only. Dosing decisions belong with a prescriber.

• Starting dose: 2.5 mg once a week for 4 weeks. This is a starting dose, not a treatment dose (FDA label).
• Increases: in 2.5 mg steps, no faster than every 4 weeks (same source; UK SmPC).
• Maintenance doses: 5, 10 or 15 mg weekly for weight and diabetes; 10 or 15 mg for sleep apnoea (US). The maximum is 15 mg a week for adults and 10 mg for children (same sources).
• Missed dose: take it within 4 days; after that, skip it and keep the usual day. Doses must be at least 3 days apart (UK SmPC).
• Injection: under the skin once a week. The UK KwikPen holds 4 doses; the MHRA says any liquid left after those 4 doses must not be extracted (MHRA). Pens must never be shared, even with a new needle (FDA label).
• NHS stopping rule: if less than 5% of starting weight is lost after 6 months at the highest tolerated dose, NICE asks clinicians to review whether to continue (NICE TA1026).

Product quality: compounded and grey-market tirzepatide

In the US, tirzepatide was on the FDA shortage list from 15 December 2022. The FDA declared the shortage over on 2 October 2024. A compounders' trade group, the Outsourcing Facilities Association, then sued, and the decision was sent back for review. On 19 December 2024 the FDA confirmed that the shortage had ended. It gave pharmacies until 18 February 2025 and outsourcing facilities until 19 March 2025 to stop making copies (FDA declaratory order). The decision was based on Lilly's own production and inventory data (FDA decision memo).

The FDA warns that compounded drugs are not reviewed for safety, effectiveness or quality. It also warns about fake labels carrying real pharmacy names, multi-dose vials kept past 28 days, and versions sold "for research purposes" (FDA). In the UK, the MHRA says genuine GLP-1 medicines come as pre-filled pens or tablets, and it has seen counterfeit pens that "closely mimic" licensed ones (MHRA). Lilly profits when copies are removed and compounders when they are allowed; the regulators' warnings are the most neutral voice.

Follow the money: who makes it and who funded the evidence

Who makes and sells it

• Eli Lilly and Company, Indianapolis, Indiana, US. Founded in 1876 and incorporated in Indiana in 1901 (Lilly 10-K). It is the only licensed maker; the EU licence is held by Eli Lilly Nederland B.V. in Utrecht (EMA) and the UK product by Eli Lilly and Company Limited, Basingstoke (UK SmPC).
• Revenue: in 2025, Mounjaro earned $22.97 billion (up 99%) and Zepbound $13.54 billion (up 175%). Total Lilly revenue was $65.18 billion, and Lilly says the two brands made up 56% of it (Lilly 10-K).
• Patents: Lilly estimates the compound patent runs to 2036 in the US, 2037 in major European countries and 2040 in Japan (same source).
• Owners: the largest shareholder is Lilly Endowment Inc., a philanthropic foundation in Indianapolis, with 9.8%. Vanguard holds 8.6%, BlackRock 6.8% and PNC 5.4% (Lilly 2026 proxy).

Who funded the evidence

Every pivotal trial we found was funded by Lilly: SURPASS-2, SURPASS-CVOT, SURMOUNT-1 and its 3-year extension, SURMOUNT-4, SURMOUNT-5, SURMOUNT-OSA and SUMMIT. Lilly employees co-wrote the papers whose author lists we checked, and several academic authors declared Lilly fees or grants. SURMOUNT-4's lead author, for example, reported grants or personal fees from Lilly, Novo Nordisk and others (SURMOUNT-4 disclosures). We found no publicly funded randomised trial of tirzepatide. The independent work is secondary: reviews that re-analyse Lilly's trials, and one publicly funded study of US insurance records. Those independent sources agree with the trials on the direction and broad size of the benefit.

A second money stream runs through competition. Novo Nordisk sells semaglutide, the main rival. Some independent reviewers have Novo ties. Two authors of the Greek review declared Novo Nordisk honoraria or advisory roles (Karagiannis 2024). Some Oxford authors are part-funded by a Novo Nordisk Foundation grant (West 2026). Neither result favours semaglutide over tirzepatide, which suggests the ties did not tilt their findings.

Documented integrity events and pricing

• UK price rise, September 2025: Lilly raised the UK private list price from 1 September 2025 to £133 for 2.5 mg and £330 for 12.5 mg and 15 mg. Pharmacies had previously paid £92 to £122. Lilly said its launch price was "significantly below the European average" and linked the change to "fair global contributions"; the rise followed US presidential complaints about US drug prices (Pharmaceutical Journal). Lilly told CNN the top-dose rise was from £122 to £330, about 170%, and that the NHS price would not change (CNN, syndicated). NICE's current list prices match the new figures (NICE TA1026, section 2). The NHS discount is "commercial in confidence".
• Product liability lawsuits: since August 2023, plaintiffs have sued Lilly and Novo Nordisk over alleged injuries from incretin drugs including Mounjaro and Zepbound. Most US cases are in two federal multidistrict litigations, one on gastrointestinal injuries and one on an eye condition (NAION). There are also class actions in Israel and Quebec (Lilly 10-K). These are allegations, not findings.
• Shortage and compounding fight: the FDA's first shortage decision was challenged in court by a compounding trade group and reissued after review (FDA order). Lilly says it will consider "filing lawsuits where appropriate" against unlawful copies (Lilly 10-K).

Related research

For wider, evidence-graded context, see these Pure City Research guides:

• Type 2 diabetes: prevention and management evidence
• Diabetes and heart disease
• Diabetes supplements: benefits, limits and safety
• Berberine: glucose evidence and safety
• Obstructive sleep apnoea
• Heart failure with preserved ejection fraction
• Acute pancreatitis
• Gallstones
• Gastroparesis
• MASLD and MASH (fatty liver)
• Myo-inositol: PCOS and fertility evidence

Frequently asked questions

Sources and funding notes

What we could not verify: the size and terms of Lilly's confidential NHS discount; the full NHS England interim commissioning guidance (we used NICE's summary of it); any publicly funded randomised trial of tirzepatide (we found none); the funding statement of the Xie 2024 network meta-analysis; tirzepatide-specific UK Yellow Card pancreatitis counts (the MHRA figures cover the whole drug class); how many people in the UK use private or illegal tirzepatide; and whether the US product-liability claims have merit (none had been decided as of Lilly's 2025 annual report). The Lilly Endowment and institutional shareholdings are as of 31 December 2025 or earlier filings and may have changed. Regulators in the US, EU and UK all assessed the same Lilly trial data, so their agreement is not fully independent confirmation.

1. FDA-approved prescribing information, Zepbound (DailyMed, revised August 2026) — warnings, side-effect rates, interactions; US regulator, text supplied by Eli Lilly.
2. Mounjaro KwikPen 10 mg SmPC (emc, revised 29 August 2026) — UK dosing, contraception, pregnancy; MHRA-approved, licence holder Eli Lilly and Company Limited.
3. EMA: Mounjaro EPAR — EU licence; European Medicines Agency.
4. openFDA Drugs@FDA: tirzepatide applications — approval dates; US government.
5. FDA news release, 8 November 2023 and 20 December 2024 — Zepbound approvals; US government.
6. NICE TA1026: Tirzepatide for managing overweight and obesity (section 2, prices; section 4, implementation) — published 23 December 2024, updated 1 September 2025; UK DHSC-sponsored public body.
7. NICE TA924: Tirzepatide for type 2 diabetes — published 25 October 2023; as above.
8. MHRA: GLP-1 medicines for weight loss and diabetes, what you need to know (updated 5 February 2026) — UK regulator.
9. MHRA press release, 5 June 2025 — contraception and pregnancy; UK regulator.
10. MHRA Drug Safety Update, 29 January 2026 — pancreatitis; UK regulator.
11. MHRA Drug Safety Update, 28 January 2025 — aspiration under anaesthesia; UK regulator.
12. MHRA news, 4 September 2024 — suicidality review; UK regulator.
13. MHRA press release, 29 December 2025 — illegal online products; UK regulator.
14. MHRA organisation page and NICE organisation page — status and sponsorship; UK government.
15. FDA Declaratory Order: Resolution of Shortages of Tirzepatide Injection Products (19 December 2024) and FDA decision memorandum — US regulator; based on Lilly's supply data.
16. FDA: Concerns with unapproved GLP-1 drugs used for weight loss — compounding risks; US regulator.
17. FDA: Prescription Drug User Fee Amendments — how industry fees fund FDA drug review.
18. Jastreboff et al. NEJM 2022, SURMOUNT-1 — pivotal obesity trial; funded by Eli Lilly.
19. Jastreboff et al. NEJM 2025, SURMOUNT-1 3-year prediabetes analysis — funded by Eli Lilly.
20. Aronne et al. NEJM 2025, SURMOUNT-5 — tirzepatide versus semaglutide; funded by Eli Lilly.
21. Aronne et al. JAMA 2024, SURMOUNT-4 — withdrawal trial; Lilly employees as authors, author disclosures as listed.
22. Frías et al. NEJM 2021, SURPASS-2 — versus semaglutide 1 mg in diabetes; funded by Eli Lilly.
23. Nicholls et al. NEJM 2025, SURPASS-CVOT — heart outcomes versus dulaglutide; funded by Eli Lilly.
24. Packer et al. NEJM 2025, SUMMIT — HFpEF; funded by Eli Lilly.
25. Malhotra et al. NEJM 2024, SURMOUNT-OSA — sleep apnoea; funded by Eli Lilly.
26. West et al. BMJ 2026 (full text) — regain after stopping; NIHR Oxford BRC; some authors part-funded by a Novo Nordisk Foundation grant.
27. Krüger et al. Nature Medicine 2026 (full text) — real-world heart outcomes; NIH and German Heart Foundation.
28. Karagiannis et al. Diabetologia 2024 (full text) — network meta-analysis in diabetes; no specific grant; two authors with Novo Nordisk ties.
29. Zeng et al. Front Endocrinol 2023 (full text) — pancreatitis and gallbladder meta-analysis; Sichuan Science and Technology Program.
30. Xie et al. Metabolism 2024 — obesity network meta-analysis; authors declared no competing interests; funding statement not checked.
31. Eli Lilly Form 10-K for 2025 (filed 12 February 2026) — revenue, patents, litigation; audited company filing.
32. Eli Lilly 2026 proxy statement — major shareholders; company filing.
33. The Pharmaceutical Journal, 18 August 2025 — UK price rise; journal of the Royal College of Pharmacy.
34. CNN (syndicated by ABC17), 15 August 2025 — Lilly's confirmation of the UK price rise; commercial news outlet.

Educational research reviewed 7 October 2026. This article is not medical advice; talk to a qualified clinician or pharmacist before starting, stopping or combining treatments.