Editorial note: this article summarises published research on tirzepatide and semaglutide. Baclabs does not sell tirzepatide, semaglutide or any peptide; we supply bacteriostatic water and reconstitution consumables. Nothing here is medical advice.
The tirzepatide vs semaglutide question has a clearer answer than most drug comparisons, because the two have been tested against each other directly. SURMOUNT-5, published in the New England Journal of Medicine in 2025, randomised 751 adults with obesity to the maximum tolerated dose of either drug for 72 weeks. Tirzepatide produced 20.2% mean weight loss; semaglutide produced 13.7% [1].
That headline hides useful detail. The two differ in receptor pharmacology, licensed dosing, tolerability and outcomes evidence, and semaglutide has cardiovascular data that tirzepatide does not yet match. This article covers the mechanism, the trial record, the UK dosing from the Summaries of Product Characteristics (SmPCs), and the practical differences. It sits under the GLP-1 peptides guide.
One receptor or two: the pharmacology
Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist. GLP-1 is released from the gut after eating; it increases glucose-dependent insulin secretion from pancreatic beta cells, suppresses glucagon, slows gastric emptying and acts on hypothalamic and brainstem appetite centres. Semaglutide is a modified version of the human hormone with a fatty diacid side chain that binds albumin and an amino acid substitution that resists breakdown by dipeptidyl peptidase-4 (DPP-4), which is what makes once-weekly dosing possible.
Tirzepatide is a single 39-amino-acid peptide that activates both the GLP-1 receptor and the receptor for glucose-dependent insulinotropic polypeptide (GIP), the other major incretin hormone. Its pharmacology was characterised by Willard and colleagues in 2020. Tirzepatide binds the GIP receptor with the same affinity as native GIP, but binds the GLP-1 receptor roughly five-fold more weakly, with about 20-fold lower potency for cyclic AMP generation. At the GLP-1 receptor it is also “biased”, recruiting beta-arrestin only weakly [2]. The authors argue this combination allows strong GIP receptor engagement while capping the GLP-1-driven gastrointestinal effects that limit dose escalation.
Semaglutide for weight loss: the STEP evidence
STEP 1 randomised 1,961 adults with overweight or obesity and no diabetes to semaglutide 2.4 mg weekly or placebo alongside lifestyle intervention. At 68 weeks the mean change in body weight was −14.9% with semaglutide and −2.4% with placebo [3].
The SELECT trial then addressed a question tirzepatide has not yet answered in a published outcomes trial: whether weight loss with the drug translates into fewer cardiovascular events. SELECT followed 17,604 people with established cardiovascular disease and a BMI of 27 or above, but without diabetes, for a mean of 39.8 months. Semaglutide reduced major adverse cardiovascular events by 20% (hazard ratio 0.80), from 8.0% to 6.5% [4]. That result is the basis for the cardiovascular risk-reduction indication in the current Wegovy SmPC [5].
Tirzepatide vs semaglutide head-to-head: SURMOUNT-1 and SURMOUNT-5
SURMOUNT-1 randomised 2,539 adults with obesity to tirzepatide 5, 10 or 15 mg weekly or placebo for 72 weeks. Mean weight loss was 16.0%, 21.4% and 22.5% respectively, versus 2.4% with placebo. In the 15 mg group, 96% lost at least 5% of body weight and 63% lost at least 20% [6].
Indirect comparison with STEP 1 suggested tirzepatide was stronger, but differing populations and estimands make such comparisons unreliable. SURMOUNT-5 settled it: adults with a BMI of 30 or above (27 with a comorbidity) and no diabetes were randomised to tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg) at the maximum tolerated dose. At 72 weeks [1]:
| Outcome at 72 weeks | Tirzepatide | Semaglutide |
|---|---|---|
| Mean weight change | −20.2% (22.8 kg) | −13.7% (15.0 kg) |
| Waist circumference change | −18.4 cm | −13.0 cm |
| Discontinuation for GI adverse events | 2.7% | 5.6% |
Tirzepatide-treated participants were more likely to reach every threshold from 10% to 25% weight loss, and men lost roughly 6% less than women in both arms. Gastrointestinal discontinuations were lower with tirzepatide despite the larger effect, consistent with its “imbalanced” pharmacology.
Licensed dosing in the UK
Both are prescription-only medicines supplied as pre-filled pens. Schedules are from the current SmPCs on medicines.org.uk.
| Wegovy (semaglutide) [5] | Mounjaro (tirzepatide) [7] | |
|---|---|---|
| Starting dose | 0.25 mg weekly for 4 weeks | 2.5 mg weekly for 4 weeks |
| Escalation | 0.5 mg, 1 mg, 1.7 mg at 4-week intervals | Increase by 2.5 mg at intervals of at least 4 weeks |
| Maintenance | 2.4 mg weekly (SmPC also lists an optional 7.2 mg dose for obesity) | 5, 10 or 15 mg weekly |
| Licensed indications | Weight management; cardiovascular risk reduction; MASH with fibrosis | Type 2 diabetes; weight management |
Ozempic, the diabetes-licensed semaglutide pen, escalates from 0.25 mg to 0.5 mg, 1 mg and a maximum of 2 mg weekly [8]. In the UK, NICE recommends tirzepatide for weight management in adults with a BMI of at least 35 kg/m² and one weight-related comorbidity, with a phased NHS rollout that began in specialist services in March 2025 and in primary care from June 2025 [9].
Adverse effects: same class, similar profile
The Wegovy SmPC reports nausea in 43.9% of trial participants, diarrhoea in 29.7%, vomiting in 24.5% and constipation in 24.2% [5]. In SURMOUNT-1, nausea affected 24.6–33.3% of tirzepatide recipients and diarrhoea 18.7–23.0%, versus 9.5% and 7.3% on placebo [6]. Most events are mild to moderate and cluster during titration.
Serious signals are shared too. In January 2026 the Medicines and Healthcare products Regulatory Agency (MHRA) strengthened pancreatitis warnings across all GLP-1 and dual GIP/GLP-1 agonists after reports of necrotising and fatal cases; treatment should stop if pancreatitis is suspected and not restart if confirmed [10]. Gallbladder disease is listed for both: cholelithiasis occurred in 1.6% of Wegovy-treated patients [5], and the Mounjaro SmPC carries an equivalent warning [7]. Semaglutide has a specific retinopathy caution in people with type 2 diabetes [8]. Both US labels carry a boxed warning about thyroid C-cell tumours seen in rodents, with contraindication in people with a history of medullary thyroid carcinoma or MEN 2 [11]; the UK SmPCs for Wegovy and Mounjaro carry no equivalent contraindication.
One difference is practical: the Mounjaro SmPC notes that tirzepatide slows gastric emptying and may reduce the absorption of oral contraceptives, particularly around initiation and dose increases [7].
Reconstitution: pens versus lyophilised vials
Everything above concerns licensed products that arrive ready to inject. A parallel market sells lyophilised tirzepatide and semaglutide vials as research chemicals; these carry none of the licensed products’ quality assurance, and the MHRA seized counterfeit tirzepatide and unlicensed peptide vials in operations during 2025 and 2026 [12]. Where a lyophilised vial is reconstituted for research, the arithmetic is the same for either peptide: concentration in mg/mL equals vial mass divided by diluent volume. A 10 mg vial with 2 mL of bacteriostatic water gives 5 mg/mL, so each unit on a U-100 insulin syringe holds 50 mcg; a 5 mg vial with 2 mL gives 2.5 mg/mL and 25 mcg per unit.
Multi-dose vials need a preserved diluent. Baclabs supplies UK-stocked bacteriostatic water in 30 mL vials; the reconstitution guide covers the general method, the GLP-1 reconstitution protocols article gives full tables for both peptides, and the storage guide explains why an opened vial is used within 28 days.
Which is better?
For weight loss alone, tirzepatide wins the only direct trial by about 6.5 percentage points at 72 weeks. Semaglutide has the deeper outcomes evidence, including a completed cardiovascular trial and a licensed indication to match. Tolerability is broadly similar. Neither result transfers to unlicensed products bearing the same names.
Frequently asked questions
Is tirzepatide better than semaglutide for weight loss?
In the direct SURMOUNT-5 trial, yes. Tirzepatide produced 20.2% mean weight loss at 72 weeks against 13.7% for semaglutide, with larger waist reductions and more participants reaching 15%, 20% and 25% thresholds [1]. Semaglutide remains highly effective, and it currently has cardiovascular outcomes evidence that tirzepatide has not yet published.
What is the difference between GIP and GLP-1?
Both are incretin hormones released from the gut after eating that boost glucose-dependent insulin secretion. GLP-1 also suppresses appetite and slows gastric emptying strongly. GIP acts on adipose tissue and the brain and, in tirzepatide’s case, appears to add weight loss and temper nausea. Semaglutide activates GLP-1 receptors only; tirzepatide activates both [2].
What is the licensed dose of semaglutide for weight loss?
Wegovy starts at 0.25 mg weekly for four weeks, then increases to 0.5 mg, 1 mg and 1.7 mg at four-week intervals before reaching a 2.4 mg weekly maintenance dose. The current UK SmPC also lists an optional 7.2 mg dose for obesity [5]. Ozempic, licensed for type 2 diabetes, tops out at 2 mg weekly [8].
Do tirzepatide and semaglutide have the same side effects?
Largely. Nausea, diarrhoea, vomiting and constipation dominate for both, mostly during dose escalation. Both carry warnings for acute pancreatitis, strengthened by the MHRA in January 2026, and gallbladder disease [10]. Semaglutide has a specific retinopathy caution in diabetes; tirzepatide’s SmPC notes reduced oral contraceptive absorption. Rodent thyroid C-cell findings apply to both classes [11].
Can you switch from semaglutide to tirzepatide?
Switching is a prescriber decision, not something the SmPCs specify in detail. Because tirzepatide has its own escalation ladder starting at 2.5 mg weekly [7], a prescriber will typically restart titration rather than map doses across. There is no published equivalence table between the two, and the pharmacology differs enough that one should not be assumed.
References
- SURMOUNT-5: Greater Loss of Weight and Waist Circumference With Tirzepatide Than Semaglutide. American College of Cardiology journal scan of Aronne LJ et al., New England Journal of Medicine, 2025. doi:10.1056/NEJMoa2416394
- Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist. Willard FS et al., JCI Insight, 2020. doi:10.1172/jci.insight.140532
- Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). Wilding JPH et al., New England Journal of Medicine, 2021. doi:10.1056/NEJMoa2032183
- Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes (SELECT). Lincoff AM et al., New England Journal of Medicine, 2023
- Wegovy FlexTouch solution for injection in pre-filled pen, Summary of Product Characteristics. Novo Nordisk, emc, 2026
- SURMOUNT-1 results published in the New England Journal of Medicine. Eli Lilly, 2022; Jastreboff AM et al., NEJM, doi:10.1056/NEJMoa2206038
- Mounjaro KwikPen 2.5 mg solution for injection in pre-filled pen, Summary of Product Characteristics. Eli Lilly, emc, 2026
- Ozempic 0.25 mg solution for injection in pre-filled pen, Summary of Product Characteristics. Novo Nordisk, emc
- Interim commissioning guidance: NICE TA1026 tirzepatide. NHS England, 2025
- GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis. MHRA Drug Safety Update, January 2026
- WEGOVY (semaglutide) injection, US Prescribing Information. FDA, 2025
- Two arrested during the MHRA’s largest ever seizure of unlicensed weight loss medicines. MHRA / GOV.UK, May 2026
Disclaimer: This content is for educational purposes only and does not constitute medical advice. Bacteriostatic water is supplied for reconstitution of substances intended for research and for use as directed by a healthcare professional. Peptides and medicines discussed here may be unlicensed in the UK or prescription-only; consult a qualified clinician before using any medicine. Baclabs does not sell peptides.