Fat Loss & Metabolic

Tirzepatide

Also known as Mounjaro, Zepbound, LY3298176, GIP/GLP-1 dual agonist, twincretin

A once-weekly injection from Eli Lilly, sold as Mounjaro and Zepbound, that is approved in the United States for type 2 diabetes, chronic weight management and obstructive sleep apnoea, and that reduced body weight by about 21 percent at 72 weeks in its main obesity trial.

How it works

A dual GLP-1/GIP agonist. Slows gastric emptying and optimizes insulin secretion for fat loss.

Performance edge

Industry leader for metabolic health; delivers weight loss results of 20% or more.

Plain-English guide

Tirzepatide, without the jargon

What it is

Tirzepatide is a laboratory-made peptide that switches on two gut hormone receptors at once: the GIP receptor (glucose-dependent insulinotropic polypeptide) and the GLP-1 receptor (glucagon-like peptide-1). Earlier drugs in this family, such as semaglutide, act on the GLP-1 receptor alone, which is why tirzepatide is sometimes called a dual agonist or a twincretin. It is given as an injection under the skin once a week. Eli Lilly developed it under the code LY3298176 and sells it as Mounjaro for type 2 diabetes and as Zepbound for weight management and sleep apnoea. Unlike most compounds on this site it is a fully approved prescription medicine with a published label, a boxed warning and a large phase 3 trial programme behind it, which means the honest comparison point for anything bought outside a pharmacy is a product that has already been tested in tens of thousands of people. Anything beyond those three approved indications is off-label, however routine it has become.

What people use it for

Blood-sugar control in adults with type 2 diabetes, the first approved indication (Mounjaro, 2022)
Chronic weight management in adults with obesity, or with overweight plus a weight-related condition (Zepbound, 2023)
Moderate to severe obstructive sleep apnoea in adults with obesity, approved in December 2024 on the strength of the SURMOUNT-OSA trials
Maintaining weight that has already been lost, which is what the SURMOUNT-4 withdrawal trial was designed to test
Cardiovascular risk in type 2 diabetes with established atherosclerotic disease, which is not an approved indication: SURPASS-CVOT compared it against another GLP-1 drug, dulaglutide, and showed only that it was not worse, which is not the same as showing a benefit against no treatment
Studied but not approved for metabolic dysfunction-associated steatohepatitis (fatty liver disease with inflammation) and for heart failure with preserved ejection fraction

How it works, simply

Think of appetite as a thermostat rather than a switch. After a meal your gut sends hormone signals that nudge the dial down for an hour or two, then it drifts back up; tirzepatide is a long-lasting copy of two of those signals at once, and one injection holds the dial lower for the whole week. The same signals slow how fast food leaves your stomach and tell the pancreas to release insulin mainly when blood sugar is actually high. So the change is not something you feel and resist at the table; it is the room already being set cooler before you walk in.

What to expect, and when

Week 1-4The label starts everyone at 2.5 mg weekly for four weeks purely to build tolerance, not for effect. Nausea, burping and constipation are most likely now and around each later step up.
Week 4-20The dose rises in 2.5 mg steps no sooner than every four weeks, up to a maintenance dose of 5, 10 or 15 mg. Appetite suppression becomes clearly noticeable and the weight curve steepens through this period.
Week 40In SURPASS-2, HbA1c had fallen by 2.01 to 2.30 percentage points depending on dose, against 1.86 with semaglutide 1 mg.
Week 72SURMOUNT-1 measured mean weight change here: minus 15.0 percent on 5 mg, minus 19.5 percent on 10 mg and minus 20.9 percent on 15 mg, against minus 3.1 percent on placebo. In people who also had type 2 diabetes (SURMOUNT-2) the same timepoint gave minus 12.8 and minus 14.7 percent.
If you stopIn SURMOUNT-4, people switched to placebo after 36 weeks regained weight: from week 36 to week 88 they gained 14.0 percent while those who stayed on tirzepatide lost a further 5.5 percent. Regain after stopping is the documented default, not a personal failure.

Side effects and interactions

Gastrointestinal effects dominate in every trial: nausea, diarrhoea, vomiting, constipation, abdominal pain, indigestion, burping and reflux, mostly mild to moderate and worst around dose increases
The US label carries a boxed warning for thyroid C-cell tumours: tirzepatide causes them in rats, and whether it does so in humans is unknown
Acute pancreatitis, which the label instructs prescribers to stop the drug for if suspected
Acute gallbladder disease, including cholecystitis, reported in the clinical trials
Acute kidney injury caused by dehydration from vomiting or diarrhoea
Low blood sugar, mainly when it is combined with insulin or a sulfonylurea; hypoglycaemia was reported in 0.2 to 1.7 percent in SURPASS-2 against 0.4 percent on semaglutide
Serious allergic reactions including anaphylaxis and angioedema, reported after marketing
Worsening of existing diabetic retinopathy, injection-site reactions, fatigue and hair loss
Food left in the stomach being breathed into the lungs during general anaesthesia or deep sedation, which is why the label tells you to warn anyone planning surgery
Suicidal behaviour and ideation are listed as a warning requiring monitoring for depression or suicidal thoughts

Who should avoid it

You are pregnant, planning a pregnancy or breastfeeding; the label says it may cause fetal harm and should be stopped when pregnancy is recognised
You or a family member has had medullary thyroid carcinoma, or you have multiple endocrine neoplasia syndrome type 2, both of which are outright contraindications on the label
You have had a serious hypersensitivity reaction to tirzepatide or any excipient in the product
You have a history of pancreatitis, gallbladder disease, severe gastroparesis (the label specifically does not recommend it there) or an active eating disorder
You have diabetic retinopathy that is proliferative or requires acute treatment; it has not been studied in that group
You are already taking another GLP-1 receptor agonist or another tirzepatide product; the label says coadministration is not recommended
You are using oral contraceptives, without adding a barrier method for four weeks after starting and after each dose increase, because delayed gastric emptying affects absorption
You are not under the supervision of a physician who can monitor blood sugar, kidney function, hydration and your other medicines

Common mistakes

Rushing the titration, or treating the maximum dose as the goal. Every trial and the label step the dose in 2.5 mg increments no sooner than every four weeks, and the gastrointestinal event rate tracks the size and speed of each step; SURMOUNT-1 showed 15.0 percent weight loss on 5 mg against 20.9 percent on 15 mg, so a lower maintenance dose that you tolerate beats a high one you abandon
Buying research-grade or compounded material now that the shortage is over. FDA ended enforcement discretion for compounded tirzepatide on 18 February 2025 for 503A pharmacies and 19 March 2025 for 503B facilities, so anything sold outside a pharmacy has no manufacturing standard behind it and the concentration in the vial is a guess
Reconstitution and storage errors: shaking the vial rather than swirling it, using the wrong diluent, drawing the wrong volume when the vial strength differs from the one the unit markings assume, or leaving reconstituted peptide out of the refrigerator or in sunlight
Stacking it with a GLP-1 receptor agonist such as semaglutide. The label explicitly does not recommend this, the pathways overlap and the gastrointestinal burden adds
Assuming the result is permanent, or ignoring what the weight is made of. SURMOUNT-4 showed 14.0 percent regain over 52 weeks after switching to placebo, so an exit plan matters as much as a start plan, and eating enough protein and doing resistance training while the weight comes off is what protects the muscle underneath it

Deep research

What the literature actually shows

Mechanism

Tirzepatide is a 39-amino-acid synthetic peptide based on the native GIP sequence (glucose-dependent insulinotropic polypeptide), modified so that it also binds the GLP-1 receptor (glucagon-like peptide-1), and acylated with a C20 fatty diacid so that it binds albumin and survives about five days in circulation. That half-life is what makes once-weekly subcutaneous dosing possible. Both targets are class B G-protein-coupled receptors, and the product label describes the drug exactly this way: a GIP receptor and GLP-1 receptor agonist.

The GLP-1 arm is the familiar one. It increases insulin release from pancreatic beta cells in a glucose-dependent way, suppresses glucagon when glucose is high, slows gastric emptying and acts in hypothalamic and brainstem appetite circuits to reduce food intake. The glucose-dependence is why hypoglycaemia is uncommon unless insulin or a sulfonylurea is also on board, and it is also why the label warns that delayed gastric emptying can change the absorption of oral medicines, including oral contraceptives.

The GIP arm is what distinguishes tirzepatide, and its contribution is still being argued about. GIP receptors sit on beta cells, on adipocytes and in the central nervous system. Proposed contributions include additional glucose-dependent insulin secretion, improved lipid handling and insulin sensitivity in fat tissue, and central effects on food intake and on nausea. Tirzepatide is an unbalanced agonist, behaving more like native GIP at the GIP receptor than like native GLP-1 at the GLP-1 receptor, and it biases GLP-1 receptor signalling toward cyclic AMP production over beta-arrestin recruitment. Whether the extra weight loss relative to semaglutide comes from GIP agonism itself or from the overall signalling profile has not been settled by any published mechanistic human study.

Clinically, the two arms together produce larger reductions in body weight and HbA1c than GLP-1 receptor agonism alone. SURPASS-2 compared it head to head with semaglutide 1 mg in type 2 diabetes and SURMOUNT-5 did so against semaglutide 2.4 mg in obesity; tirzepatide was superior on the primary endpoint in both. The downstream effects follow the weight and glycaemic change rather than being separate actions: apnoea-hypopnoea index in SURMOUNT-OSA, liver histology in the phase 2 MASH trial, and cardiovascular events in SURPASS-CVOT, where the comparison was against another GLP-1 receptor agonist rather than placebo.

Strength of evidence

Human trialsTirzepatide itself has been tested in multiple large randomised controlled trials in humans, including placebo-controlled obesity trials, two head-to-head trials against semaglutide, a randomised withdrawal trial and a 13,299-participant cardiovascular outcomes trial, all using the same once-weekly subcutaneous route as the cheatsheet protocol; the cheatsheet 2.5 mg starting dose also matches the approved label, although the trial results above come from maintenance doses of 5 to 15 mg reached by titration.

Key studies

Randomised controlled trial2022New England Journal of Medicinen = 2539
Tirzepatide Once Weekly for the Treatment of Obesity
SURMOUNT-1, a 72-week phase 3 double-blind placebo-controlled trial in adults with a BMI of 30 or more, or 27 or more with a weight-related complication and without diabetes. Mean weight change at week 72 was minus 15.0 percent on 5 mg, minus 19.5 percent on 10 mg and minus 20.9 percent on 15 mg, against minus 3.1 percent on placebo; 50 percent (10 mg) and 57 percent (15 mg) lost at least 20 percent of body weight against 3 percent on placebo. Adverse events were mostly mild-to-moderate gastrointestinal events, and discontinuation for adverse events ran 4.3 to 7.1 percent against 2.6 percent on placebo.
Randomised controlled trial2021New England Journal of Medicinen = 1879
Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes
SURPASS-2, a 40-week open-label phase 3 head-to-head trial against semaglutide 1 mg. HbA1c fell by 2.01, 2.24 and 2.30 percentage points on tirzepatide 5, 10 and 15 mg against 1.86 on semaglutide, and tirzepatide produced 1.9 to 5.5 kg more weight loss. Nausea occurred in 17 to 22 percent on tirzepatide against 18 percent on semaglutide; hypoglycaemia below 54 mg/dL occurred in 0.2 to 1.7 percent against 0.4 percent, and serious adverse events in 5 to 7 percent against 3 percent.
Randomised controlled trial2023The Lancetn = 938
Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2): a double-blind, randomised, multicentre, placebo-controlled, phase 3 trial
In adults with both obesity and type 2 diabetes, mean body-weight change at week 72 was minus 12.8 percent on 10 mg and minus 14.7 percent on 15 mg, against minus 3.2 percent on placebo; 79 to 83 percent lost at least 5 percent of body weight against 32 percent on placebo. Weight loss in this population was smaller than in SURMOUNT-1, where participants did not have diabetes. Adverse events were predominantly gastrointestinal, mostly mild to moderate, with fewer than 5 percent discontinuing because of them.
Randomised controlled trial2024JAMAn = 670
Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial
After a 36-week open-label lead-in on maximum tolerated tirzepatide, 670 participants were randomised to continue the drug or switch to placebo for 52 more weeks. Mean weight change from week 36 to week 88 was minus 5.5 percent on continued tirzepatide against plus 14.0 percent on placebo, a difference of 19.4 percentage points. At week 88, 89.5 percent of those continuing treatment had kept at least 80 percent of their lead-in weight loss, against 16.6 percent on placebo.
Randomised controlled trial2025New England Journal of Medicinen = 751
Tirzepatide as Compared with Semaglutide for the Treatment of Obesity
SURMOUNT-5, a 72-week open-label phase 3b head-to-head trial in adults with obesity and without diabetes, comparing maximum tolerated tirzepatide with maximum tolerated semaglutide. Mean weight change at week 72 was minus 20.2 percent on tirzepatide against minus 13.7 percent on semaglutide, and waist circumference fell 18.4 cm against 13.0 cm. Gastrointestinal events were the most common adverse events in both groups and were mostly mild to moderate.
Randomised controlled trial2025New England Journal of Medicinen = 13299
Cardiovascular Outcomes with Tirzepatide versus Dulaglutide in Type 2 Diabetes
SURPASS-CVOT, a double-blind active-comparator noninferiority phase 3 trial in people with type 2 diabetes and atherosclerotic cardiovascular disease. The composite of cardiovascular death, myocardial infarction or stroke occurred in 801 of 6586 patients (12.2 percent) on tirzepatide and 862 of 6579 (13.1 percent) on dulaglutide, hazard ratio 0.92, meeting noninferiority. More gastrointestinal adverse events occurred on tirzepatide. Because the comparator was an active GLP-1 receptor agonist rather than placebo, this trial shows non-inferiority to an established drug, not a placebo-controlled cardiovascular benefit.
Randomised controlled trial2024New England Journal of Medicinen = 469
Tirzepatide for the Treatment of Obstructive Sleep Apnea and Obesity
Two 52-week phase 3 double-blind placebo-controlled trials in adults with obesity and moderate-to-severe obstructive sleep apnoea, one in people using positive airway pressure and one in people not using it. The apnoea-hypopnoea index fell by 25.3 events per hour against 5.3 on placebo in the first trial, and by 29.3 against 5.5 in the second. Adverse events were mostly mild-to-moderate gastrointestinal events. These trials supported the FDA approval of Zepbound for obstructive sleep apnoea in December 2024.
Randomised controlled trial2024New England Journal of Medicinen = 190
Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis
A 52-week phase 2 dose-finding double-blind placebo-controlled trial in biopsy-confirmed metabolic dysfunction-associated steatohepatitis with stage F2 or F3 fibrosis; 157 participants had evaluable week-52 biopsies. Resolution of steatohepatitis without worsening of fibrosis occurred in 10 percent on placebo against 44, 56 and 62 percent on tirzepatide 5, 10 and 15 mg. The trial was not powered for fibrosis improvement, and tirzepatide is not approved for this indication.

Safety data

The safety picture for tirzepatide is unusually well characterised because it comes from an approved label rather than from inference. The US prescribing information carries a boxed warning for thyroid C-cell tumours: tirzepatide causes them in rats, and whether it does the same in humans is unknown. It is contraindicated outright in anyone with a personal or family history of medullary thyroid carcinoma or with multiple endocrine neoplasia syndrome type 2, and in anyone with a known serious hypersensitivity to the drug or its excipients.

Beyond the boxed warning, the label lists severe gastrointestinal reactions (and does not recommend use in severe gastroparesis), acute kidney injury from volume depletion, acute gallbladder disease, acute pancreatitis, serious hypersensitivity reactions including anaphylaxis and angioedema reported after marketing, hypoglycaemia when combined with insulin or an insulin secretagogue, progression of existing diabetic retinopathy, suicidal behaviour and ideation, and pulmonary aspiration during general anaesthesia or deep sedation. The commonest adverse reactions at 5 percent or more are nausea, diarrhoea, vomiting, constipation, abdominal pain, indigestion, injection-site reactions, fatigue, hypersensitivity reactions, burping, hair loss and reflux. In the trials, discontinuation for adverse events ran 4.3 to 7.1 percent in SURMOUNT-1 against 2.6 percent on placebo. In pregnancy the label says it may cause fetal harm and should be discontinued.

A separate risk attaches to supply. Because tirzepatide is an approved drug that is no longer in shortage, FDA ended enforcement discretion for compounded copies on 18 February 2025 for 503A pharmacies and 19 March 2025 for 503B outsourcing facilities, and treats compounded GLP-1 products containing the same active ingredient at a comparable strength and route as essentially copies of the approved drug. Material sold as research-grade tirzepatide outside the pharmacy system carries none of the label, contraindication screening, pharmacovigilance or manufacturing controls that produced the safety data summarised above, and its actual content is not verified by anyone.

Regulatory status

FDA: As of 19 September 2026 tirzepatide is FDA-approved: as Mounjaro for type 2 diabetes since 2022, and as Zepbound for chronic weight management since 8 November 2023 and for moderate-to-severe obstructive sleep apnoea in adults with obesity since 20 December 2024; the label carries a boxed warning for thyroid C-cell tumours, and FDA ended enforcement discretion for compounded tirzepatide in February and March 2025 after the shortage was declared resolved.
WADA: Tirzepatide is not named anywhere on the WADA 2026 Prohibited List, and as an approved therapeutic medicine it does not fall under section S0 (non-approved substances); it is not an insulin or insulin-mimetic under S4.4.2, so it is not prohibited in or out of competition, though athletes should confirm current status with their anti-doping organisation because GLP-1 drugs have been discussed for the Monitoring Programme.
Source: https://www.fda.gov/news-events/press-announcements/fda-approves-first-medication-obstructive-sleep-apnea

Open questions

How much of the advantage over semaglutide comes from GIP receptor agonism itself rather than from the overall signalling profile or the higher achievable exposure? SURPASS-2 and SURMOUNT-5 show that tirzepatide wins, but no published human mechanistic study separates the two receptor arms.
Is there a placebo-controlled cardiovascular benefit? SURPASS-CVOT was designed against dulaglutide, an active GLP-1 receptor agonist, and demonstrated noninferiority with a hazard ratio of 0.92, so the size of any benefit over no treatment is inferred rather than measured.
What is the human relevance of the rodent thyroid C-cell tumours behind the boxed warning? The label states plainly that this is unknown, and long-term post-marketing surveillance has not resolved it.
How much of the weight lost is lean mass, and what does that mean over many years? Body-composition data exist from substudies, but the long-term functional consequences of repeated large weight swings, including the 14.0 percent regain seen in SURMOUNT-4 after withdrawal, are not established.
Does the phase 2 liver histology result translate into hard outcomes? The MASH trial showed steatohepatitis resolution in 44 to 62 percent against 10 percent on placebo, but it was not powered for fibrosis improvement and the drug is not approved for that use.