Tirzepatide: what it is, side effects, dosage and the research

Tirzepatide is a synthetic peptide that acts on the receptors for two incretin hormones: glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). It carries a fatty-acid modification that lets it circulate for about a week. Eli Lilly first reported it in 2018 under the code LY3298176, describing a molecule designed to test whether adding GIP activity improves on selective GLP-1 drugs such as semaglutide ^[1].

It is sold as two branded medicines: Mounjaro, approved in the US in May 2022 for type 2 diabetes, and Zepbound, approved in the US in November 2023 for chronic weight management and in December 2024 for moderate-to-severe obstructive sleep apnoea in adults with obesity. In the UK and EU the Mounjaro brand is used for both diabetes and weight management. Both come as prefilled pens or vials made to pharmaceutical standards and supplied on prescription.

Tirzepatide is also sold online as a freeze-dried powder labelled for research use. That product is made by unidentified manufacturers and is not Mounjaro or Zepbound. Tirzepatide sits between semaglutide (GLP-1 only) and the investigational triple agonist retatrutide, which adds glucagon receptor activity.

Yes. The US label describes tirzepatide as based on the sequence of the hormone GIP, with the unnatural amino acid aminoisobutyric acid (Aib) at positions 2 and 13 and a lysine at position 20 attached through a linker to a 20-carbon fatty diacid. Its molecular weight is 4813.53 Da. The fatty acid binds albumin, and the label reports that tirzepatide is 99% bound to albumin in plasma, with 80% of a subcutaneous dose reaching the bloodstream ^[10].

Is Zepbound the same as tirzepatide? Yes: Zepbound and Mounjaro are both brand names for tirzepatide, made by Eli Lilly ^{[10] [11]}. Neither contains semaglutide. In the US, Mounjaro is indicated for blood-glucose control in adults and children aged 10 and over with type 2 diabetes, and to lower the risk of major cardiovascular events in adults with type 2 diabetes and established heart disease ^[11]. Zepbound is indicated for weight reduction and long-term maintenance and for obstructive sleep apnoea in adults with obesity ^[10].

At the GLP-1 receptor, tirzepatide has the effects familiar from other GLP-1 drugs: glucose-dependent insulin release, lower glucagon, slower stomach emptying and reduced appetite. Its GIP activity is the distinguishing feature. In Lilly’s discovery work, tirzepatide activated both receptors in cells, and in mice it reduced body weight and food intake more than a selective GLP-1 agonist did ^[1].

Exactly how GIP activity adds to the effect in people is still debated, but the clinical result is consistent: in direct comparisons, tirzepatide lowered blood glucose and body weight more than semaglutide ^{[5] [6]}.

SURMOUNT-1 was the pivotal obesity trial. It randomised 2,539 adults with a BMI of 30 or more (or 27 or more with a weight-related complication), excluding diabetes, to tirzepatide 5 mg, 10 mg or 15 mg once weekly, or placebo, for 72 weeks. Mean weight change was −15.0%, −19.5% and −20.9% on the three doses against −3.1% on placebo. At least 5% weight loss was reached by 85%, 89% and 91% of participants on the three doses, against 35% on placebo, and at the 15 mg dose 57% of participants lost 20% or more of their body weight. All prespecified cardiometabolic measures, such as waist circumference, blood pressure and lipids, improved with tirzepatide ^[2].

SURMOUNT-5 compared tirzepatide head-to-head with semaglutide in 751 adults with obesity but without type 2 diabetes. At the maximum tolerated doses, weight fell by 20.2% with tirzepatide and 13.7% with semaglutide over 72 weeks, and waist circumference fell by 18.4 cm against 13.0 cm ^[6].

SURMOUNT-4 tested what happens when tirzepatide is stopped. All participants took the maximum tolerated dose (10 mg or 15 mg) for a 36-week lead-in, during which average weight fell by 20.9%. They were then randomised to continue tirzepatide or switch to placebo. Between weeks 36 and 88, those who continued lost a further 5.5%, while those switched to placebo regained 14.0%. Over the whole 88 weeks, weight change was −25.3% on continued tirzepatide and −9.9% in the group switched to placebo ^[12]. As with semaglutide, much of the lost weight returned after stopping.

All of these trials combined tirzepatide with a reduced-calorie diet and increased physical activity, which is also how the US label frames the weight indication ^[10].

In type 2 diabetes, SURPASS-1 tested tirzepatide alone against placebo in 478 people: after 40 weeks HbA1c had fallen by 1.87 to 2.07 percentage points on 5 to 15 mg, against a small rise on placebo ^[7]. SURPASS-2 compared it with semaglutide 1 mg in 1,879 people over 40 weeks. HbA1c fell by 2.01 to 2.30 percentage points with tirzepatide 5 to 15 mg against 1.86 points with semaglutide, and every tirzepatide dose was superior on this measure. Weight reductions were also greater than with semaglutide, by 1.9 kg, 3.6 kg and 5.5 kg on the three doses. Rates of nausea were similar: 17% to 22% with tirzepatide and 18% with semaglutide ^[5].

SURMOUNT-OSA comprised two 52-week trials in adults with moderate-to-severe obstructive sleep apnoea and obesity. Tirzepatide reduced the apnoea–hypopnoea index by about 20 to 24 more events per hour than placebo, in people both without and with CPAP-type therapy ^[3]. These results supported the US approval for sleep apnoea.

SUMMIT randomised 731 people with heart failure with preserved ejection fraction and obesity. Over a median follow-up of 104 weeks, cardiovascular death or worsening heart failure occurred in 9.9% on tirzepatide against 15.3% on placebo, and symptoms and physical limitations improved more with tirzepatide ^[4].

SURPASS-CVOT, described on the US Mounjaro label, compared tirzepatide with the older GLP-1 drug dulaglutide 1.5 mg in 13,299 adults with type 2 diabetes and heart disease, over a median of 210.1 weeks. The hazard ratio for major cardiovascular events was 0.92 (95.3% confidence interval 0.83 to 1.01). Tirzepatide met the test for non-inferiority, but superiority over dulaglutide was not shown ^[11].

Tirzepatide vs semaglutide is the comparison most people search for, and it is one of the few in this class settled by randomised head-to-head trials. The main difference is pharmacological: semaglutide activates only the GLP-1 receptor, while tirzepatide activates both GIP and GLP-1 receptors ^[1]. Our Tirzepatide vs semaglutide comparison sets the two side by side.

In obesity without diabetes, SURMOUNT-5 found 20.2% average weight loss with tirzepatide against 13.7% with semaglutide at 72 weeks ^[6]. In type 2 diabetes, SURPASS-2 found larger HbA1c and weight reductions with all three tirzepatide doses than with semaglutide 1 mg, with similar rates of nausea ^[5]. These results describe average outcomes in trial populations; the choice between two licensed medicines for a particular person is a clinical decision.

Switching from semaglutide to tirzepatide was not a studied protocol in these trials, and the labels do not give a conversion between the two. The Zepbound label states that using it with another GLP-1 receptor agonist is not recommended ^[10]. The UK regulator advises that people speak to a healthcare professional before switching between GLP-1 medicines, because they differ in strength and in how they are used ^[15].

Retatrutide is an investigational drug that adds glucagon receptor activity to the GIP and GLP-1 activity of tirzepatide. It is not approved anywhere we could confirm, so any comparison is between a licensed medicine and an experimental one. Our Retatrutide vs tirzepatide comparison sets the two side by side.

No head-to-head result has been published. In separate trials, retatrutide 12 mg produced an average weight change of −24.2% at 48 weeks in a phase 2 study ^[18], and tirzepatide 15 mg produced −20.9% at 72 weeks in SURMOUNT-1 ^[2]. Comparing numbers across different trials, populations and durations is unreliable. A direct phase 3 comparison, TRIUMPH-5, is registered: about 800 adults with obesity are randomised to retatrutide or tirzepatide, with the primary outcome at week 80. As of the registry record we checked, it had no posted results ^[19].

Combinations are also studied only within trials. Novo Nordisk has compared CagriSema, a combination of semaglutide with the amylin analogue cagrilintide, against tirzepatide 15 mg in an 84-week trial that had no posted results when we checked ^[21].

There are no approved tirzepatide pills. Tirzepatide is licensed only as a subcutaneous injection, in pens and vials ^[10]. Products sold online as oral tirzepatide, drops or capsules are not approved medicines, and no published trial has tested such a product.

Searches for oral tirzepatide often mean Lilly’s newer tablet, orforglipron, which the US FDA approved in 2026 under the brand name Foundayo as an oral GLP-1 receptor agonist for weight management ^[16]. Orforglipron is a different molecule: a small non-peptide drug that acts at the GLP-1 receptor, with a molecular weight of about 902 g/mol for its calcium salt against 4,813 for tirzepatide, and it is not a tablet form of tirzepatide ^{[16] [17]}. So Foundayo vs tirzepatide is a comparison of two different drugs.

In the ATTAIN-1 trial of 3,127 adults with obesity, orforglipron at 6 mg, 12 mg or 36 mg daily produced average weight changes of −7.5%, −8.4% and −11.2% at 72 weeks, against −2.1% with placebo ^[17]. This was not a head-to-head comparison with tirzepatide.

Tirzepatide has a half-life of about 5 days, and the label states that steady-state levels are reached after about 4 weeks of weekly dosing ^[10]. The dose is then increased in 2.5 mg steps at intervals of at least 4 weeks, so reaching 15 mg takes at least 20 weeks.

In the trials, weight change built up over months. By the end of the 36-week lead-in in SURMOUNT-4, average loss was 20.9% ^[12]. In SURMOUNT-1, the final averages of 15.0% to 20.9% were measured at 72 weeks ^[2]. HbA1c results in diabetes trials were measured at 40 weeks ^{[5] [7]}. Individual responses varied: on 15 mg in SURMOUNT-1, 9% of participants did not reach 5% weight loss ^[2].

Tirzepatide’s evidence base is large: many thousands of participants across randomised, mostly placebo-controlled phase 3 trials, many published in the New England Journal of Medicine and The Lancet. As with semaglutide, this evidence describes the licensed product used as prescribed. It is not evidence about unlicensed powders, whose content and purity are unknown.

SURMOUNT-1 used 5 mg, 10 mg or 15 mg by subcutaneous injection once weekly, with a 20-week dose-escalation period before the higher doses were reached ^[2]. SURPASS-2 used the same three maintenance doses ^[5]. SURMOUNT-OSA and SURMOUNT-5 used the maximum tolerated dose, 10 mg or 15 mg weekly ^{[3] [6]}, and SUMMIT used doses up to 15 mg weekly ^[4]. Lilly’s phase 1 work explored single doses of 0.25 to 8 mg and multiple doses up to 15 mg ^[1].

These are the doses used in trials of the licensed medicine, reported for information, not as a recommendation. They cannot be applied to research-grade vials whose true peptide content has not been verified. Our peptide calculator covers the arithmetic of reconstitution only.

The dosage chart people search for is the escalation schedule in the prescribing information. On the US Zepbound label, treatment starts at 2.5 mg once weekly for 4 weeks. The label states that 2.5 mg is for starting treatment only and is not a maintenance dose. The dose then rises in 2.5 mg steps after at least 4 weeks on each dose. The maintenance doses are 5 mg, 10 mg or 15 mg weekly for weight reduction and 10 mg or 15 mg for sleep apnoea, and the maximum is 15 mg once weekly ^[10]. Mounjaro follows the same pattern for type 2 diabetes ^[11].

The labels describe injection under the skin of the abdomen or thigh, or the back of the upper arm if someone else gives it, on the same day each week, rotating the site. If a dose is missed, it can be taken within 4 days (96 hours); after that it is skipped, and doses must be at least 3 days apart ^{[10] [11]}.

Tirzepatide dosage in units comes up because licensed pens are set in milligrams, while compounded and research vials are drawn with insulin syringes. On a U-100 syringe 100 units equal 1 mL, so the milligrams in a given number of units depend on the vial’s concentration. At 10 mg per mL, 25 units (0.25 mL) contain 2.5 mg; at 5 mg per mL, the same 25 units contain 1.25 mg. The FDA reports adverse events linked to dosing errors with compounded GLP-1 drugs, including errors by clinicians calculating doses ^[13]. Our peptide calculator and bacteriostatic water guide explain the arithmetic.

Microdosing tirzepatide, meaning regular use of doses below the labelled schedule, has not been tested in any randomised trial we found, and the label does not treat 2.5 mg as a maintenance dose ^[10].

The most common side effects were gastrointestinal (nausea, diarrhoea, vomiting and constipation), mostly mild to moderate and concentrated in the dose-escalation period. In SURMOUNT-1, adverse events led to stopping treatment in 4.3%, 7.1% and 6.2% of participants on 5, 10 and 15 mg, against 2.6% on placebo ^[2]. In SUMMIT, 6.3% stopped tirzepatide because of adverse events, mainly gastrointestinal, against 1.4% on placebo ^[4].

In SURPASS-2, serious adverse events were reported in 5% to 7% of tirzepatide users and 3% of semaglutide users, and low blood glucose (below 54 mg/dL) was uncommon in both groups ^[5]. Prescribing information for tirzepatide, like other GLP-1-based medicines, also lists risks such as pancreatitis and gallbladder disease, and the US label carries a boxed warning about thyroid C-cell tumours seen in rodents.

For unlicensed products, the risks from uncertain dose, impurities and contamination are added to the drug’s known side effects. Testing of research-grade GLP-1 peptides bought online has found large deviations from the labelled content (see our semaglutide and retatrutide guides for published examples).

The US Zepbound label pools the placebo-controlled weight trials. Nausea was reported by 25%, 29% and 28% of people on 5, 10 and 15 mg against 8% on placebo; diarrhoea by 19% to 23% against 8%; vomiting by 8% to 13% against 2%; and constipation by 11% to 17% against 5%. Gastrointestinal side effects overall affected 56% on each dose against 30% on placebo, mostly during dose escalation. Injection-site reactions were reported by 6% to 8% against 2% ^[10].

Tirzepatide hair loss was reported by 4% to 5% on the three doses against 1% on placebo, and was more common in women (7.1%) than men (0.5%). Fatigue was reported by 5% to 7% against 3% on placebo, so tiredness was more frequent on tirzepatide but affected a minority. Dizziness was reported by 4% to 5% against 2% ^[10].

Headache and anxiety do not appear in the label’s table of common side effects in the weight trials. Both are listed in the Medication Guide as possible signs of low blood sugar, which is mainly a risk when tirzepatide is used with insulin or a sulfonylurea ^[10].

Two points concern contraception and pregnancy. Tirzepatide can reduce the effect of oral contraceptive pills, and the label advises switching to a non-oral method or adding a barrier method for 4 weeks after starting and after each dose increase ^[10]. The UK regulator gives the same 4-week advice and says tirzepatide should be stopped at least 1 month before a planned pregnancy ^[15].

Prescribing information also warns of pancreatitis, gallbladder disease and, in rodents, thyroid C-cell tumours. In the weight trials, adjudicated pancreatitis was reported in 0.2% on tirzepatide and 0.2% on placebo ^[10].

Compounded tirzepatide is tirzepatide mixed by a pharmacy rather than made by Lilly. It became widely available in the US while tirzepatide injection was on the FDA’s shortage list. The FDA declared that shortage resolved on 19 December 2024, and the period in which it did not intend to act against state-licensed pharmacies making copies ended on 18 February 2025 ^[14]. On 30 April 2026 the FDA proposed excluding tirzepatide, semaglutide and liraglutide from the list of bulk substances outsourcing facilities may use; we found no final decision ^[20].

The FDA states that compounded drugs are not FDA approved and are not reviewed for safety, effectiveness or quality before marketing. It had received more than 730 adverse event reports associated with compounded tirzepatide by 31 May 2026, and has warned about fraudulent compounded products and about tirzepatide sold as “for research purposes” ^[13]. No randomised trial has tested a compounded tirzepatide product, so the trial results above cannot be assumed to apply to one. Our guide to what research peptides are explains how that label differs from a medicine.

Yes. The FDA approved tirzepatide as Mounjaro for type 2 diabetes in May 2022, as Zepbound for chronic weight management in November 2023, and as Zepbound for obstructive sleep apnoea in adults with obesity in December 2024 ^{[8] [9]}. The approval covers Lilly’s products only. Compounded and research-labelled tirzepatide are not FDA approved; see the US legal status page.

Tirzepatide is a licensed prescription-only medicine. In the US, Mounjaro is approved for type 2 diabetes (May 2022) and Zepbound for chronic weight management (November 2023) ^[8] and for obstructive sleep apnoea in adults with obesity (December 2024) ^[9]. It is also licensed in the UK, the EU and other markets, where Mounjaro is used for both diabetes and weight management.

Unlicensed tirzepatide sold as a “research chemical” is not covered by any of these approvals. How the law treats its sale and possession differs between countries; see our legal status overview and the UK page.

Tirzepatide in the UK is licensed as Mounjaro for both diabetes and weight management. The MHRA says all GLP-1 medicines are prescription-only, that some are available on the NHS depending on location, and that selling them through unregulated channels such as beauty salons or social media is against the law. It also states that products supplied as a powder in vials to be mixed before injection are not authorised ^[15].

Licensed tirzepatide pens and vials are stored in a refrigerator and must not be frozen; the product leaflet states how long they can be kept unrefrigerated. Research-grade tirzepatide is usually supplied freeze-dried and is generally kept cold, dry and dark, and refrigerated after reconstitution. See how to store peptides.

Does tirzepatide need to be refrigerated? The US label says to store it at 2 °C to 8 °C and not to freeze it. If needed, a single-dose pen or vial can be kept unrefrigerated at up to 30 °C for a total of 21 days. The multi-dose vial and KwikPen can be kept in the fridge or at room temperature up to 30 °C after first use, for up to 30 days ^[10]. The FDA has received complaints about compounded GLP-1 products arriving warm, and advises that multi-dose vials of compounded products are discarded 28 days after first use ^[13].

With a half-life of about 5 days ^[10], most of a dose is cleared within a few weeks of the last injection, which is why the UK advice on stopping before pregnancy uses a one-month gap ^[15].

PepFinder does not sell tirzepatide. To compare research suppliers, see compare prices, read how to read a COA, and check which vendors provide third-party tested batches. A certificate from an independent laboratory confirming identity and quantity matters more than price.

Research-labelled tirzepatide listed on tirzepatide UK prices is not Mounjaro, and the MHRA says powder vials are not an authorised form ^[15]. Our guides to third-party peptide testing and spotting a fake peptide supplier cover what a real test report shows.