The weight loss medication landscape is evolving at a remarkable pace. In just a few years, we have moved from semaglutide (the GLP-1 agonist behind Ozempic and Wegovy) to tirzepatide (the dual agonist sold as Mounjaro and Zepbound), and now to retatrutide, a triple-receptor agonist that has produced the most striking weight loss results ever recorded in a clinical trial.
If you are considering weight loss medication, or already taking one and wondering what comes next, this guide offers a thorough, evidence-based comparison of retatrutide vs semaglutide vs tirzepatide. We cover how each drug works, what the clinical trials actually show, side effects, costs, availability in the UK, and crucially, the blood tests you should be having regardless of which medication you use.
Key Takeaways
- Semaglutide (Wegovy/Ozempic) targets one receptor (GLP-1) and delivers roughly 15% body weight loss. It is the only one with proven cardiovascular benefits.
- Tirzepatide (Mounjaro/Zepbound) targets two receptors (GLP-1 + GIP) and achieves approximately 20–22% weight loss. Head-to-head, it beat semaglutide by 47% in the SURMOUNT-5 trial.
- Retatrutide targets three receptors (GLP-1 + GIP + glucagon) and has delivered up to 28.7% weight loss in the Phase 3 TRIUMPH-4 trial, along with extraordinary liver fat reduction of up to 86%.
- Semaglutide and tirzepatide are available now in the UK. Retatrutide is still in Phase 3 trials and may receive MHRA approval in late 2027 or 2028.
- Blood testing is essential with any weight loss medication to monitor liver function, blood sugar, thyroid health, and nutritional status.
The Three Generations of Weight Loss Injections
To understand the difference between these medications, it helps to see them as three generations of the same scientific idea: harnessing gut hormones to control appetite, blood sugar, and metabolism.
Generation 1 — Semaglutide (2017/2021): The breakthrough. Semaglutide mimics a single gut hormone called GLP-1 (glucagon-like peptide-1). Originally developed for type 2 diabetes as Ozempic, it was later approved at a higher dose for weight management as Wegovy. It proved that targeting GLP-1 alone could produce meaningful, sustained weight loss far beyond anything previously achieved with medication.
Generation 2 — Tirzepatide (2022/2023): The upgrade. Tirzepatide acts on two receptors simultaneously: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). This dual mechanism, marketed as Mounjaro for diabetes and Zepbound for weight loss, pushed efficacy significantly higher. The landmark HbA1c reductions and weight loss figures exceeded semaglutide by a substantial margin.
Generation 3 — Retatrutide (investigational): The triple threat. Retatrutide adds a third receptor to the mix: the glucagon receptor. This addition appears to unlock effects that neither semaglutide nor tirzepatide can match, particularly in liver fat reduction and total metabolic improvement. It is manufactured by Eli Lilly and currently in Phase 3 trials under the TRIUMPH programme.
How Each Drug Works: Mechanism Comparison
All three medications are injectable peptides administered once weekly by subcutaneous injection. The fundamental difference lies in how many hormonal pathways each one activates.
GLP-1 is the shared foundation across all three drugs. It slows gastric emptying (making you feel full longer), reduces appetite signals in the brain, and stimulates insulin secretion when blood sugar is elevated. This is why all three drugs work for both weight loss and type 2 diabetes.
GIP (added in tirzepatide and retatrutide) works synergistically with GLP-1 to amplify insulin secretion, improve fat metabolism, and enhance satiety. The combination of GLP-1 and GIP appears to produce greater weight loss than GLP-1 alone.
Glucagon receptor activation (unique to retatrutide) is the most intriguing addition. Glucagon is traditionally viewed as a counter-regulatory hormone that raises blood sugar, but when combined with GLP-1 and GIP agonism, its effects on energy expenditure, fat oxidation, and hepatic lipid metabolism become powerful tools. This is likely why retatrutide produces such dramatic liver fat reduction.
Head-to-Head Comparison Table
The following table summarises the key differences based on published clinical trial data and current market information.
| Category | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| Mechanism | GLP-1 agonist | GLP-1 + GIP dual agonist | GLP-1 + GIP + Glucagon triple agonist |
| Brand Names | Ozempic, Wegovy, Rybelsus | Mounjaro, Zepbound | Not yet branded (investigational) |
| Weight Loss (max dose) | ~14.9% (68 wk, STEP 1) | ~22.5% (72 wk, SURMOUNT-1) | ~28.7% (68 wk, TRIUMPH-4) |
| Head-to-Head (SURMOUNT-5) | 13.7% at 72 weeks | 20.2% at 72 weeks | Not yet compared head-to-head |
| HbA1c Reduction | ~1.5–1.8% | ~2.0–2.4% | ~2.2% (Phase 2) |
| Liver Fat Reduction | Modest (~15–20%) | Significant (MASH resolution in 62%) | Dramatic (~86% at 48 wk; 93% normalised) |
| Cardiovascular Data | 20% MACE reduction (SELECT trial) | Trials ongoing | No outcomes data yet |
| Dosing | Once weekly (up to 2.4 mg) | Once weekly (up to 15 mg) | Once weekly (up to 12 mg) |
| GI Side Effects | Nausea ~44%, vomiting ~24% | Nausea ~33%, vomiting ~12% | Nausea ~43%, vomiting ~21% |
| Unique Safety Signal | Pancreatitis risk (rare) | Gallbladder events | Dysesthesia (up to 20.9%) |
| UK Availability | Available (NHS limited; private) | Available (NHS very limited; private) | Not available (est. 2027–2028) |
| Private Cost (UK, monthly) | ~£130–£295 | ~£149–£375 | TBC (not yet available) |
| Monitoring Required | Blood sugar, thyroid, liver, kidneys | Blood sugar, thyroid, liver, kidneys | Blood sugar, thyroid, liver, kidneys (likely expanded panel) |
Weight Loss Results Compared: What the Trials Show
Semaglutide: The STEP Trials
The STEP 1 trial, published in the New England Journal of Medicine, enrolled 1,961 adults with obesity or overweight with at least one weight-related comorbidity. Participants receiving semaglutide 2.4 mg weekly lost an average of 14.9% of body weight over 68 weeks, compared to 2.4% with placebo. Approximately one-third of participants lost 20% or more of their body weight, a threshold once thought achievable only through bariatric surgery.
Tirzepatide: The SURMOUNT Trials
SURMOUNT-1 showed tirzepatide producing weight loss of 15%, 19.5%, and 22.5% at the 5 mg, 10 mg, and 15 mg doses respectively over 72 weeks. The higher doses helped over half of participants achieve a 20% or greater reduction in body weight.
More compellingly, the head-to-head SURMOUNT-5 trial directly compared tirzepatide with semaglutide over 72 weeks. The results, published in the New England Journal of Medicine in 2025, were decisive: tirzepatide achieved 20.2% body weight loss versus 13.7% for semaglutide. That is a 47% greater relative weight loss with tirzepatide. Among participants on tirzepatide, 31.6% lost at least 25% of their body weight, compared to 16.1% on semaglutide.
Retatrutide: The TRIUMPH Programme
Retatrutide raised the bar further. In the Phase 2 trial published in the NEJM in 2023, the highest dose (12 mg) produced 24.2% body weight loss over 48 weeks. The Phase 3 TRIUMPH-4 trial, reported in late 2025, confirmed and exceeded these results: the 12 mg dose achieved 28.7% average body weight loss (approximately 71.2 lbs or 32 kg) at 68 weeks. The 9 mg dose achieved 26.4%.
To put this in perspective, if you weigh 100 kg (15 stone 10 lbs), retatrutide could potentially reduce your weight to approximately 71 kg (11 stone 2 lbs) in just over a year. This rivals the results of some bariatric surgery procedures.
Typical Weight Loss at Maximum Dose (Clinical Trial Averages)
Data from STEP 1 (68 wk), SURMOUNT-1 (72 wk), and TRIUMPH-4 (68 wk). Maximum approved or trialled dose for each drug. Results are population averages; individual results vary.
Taking a Weight Loss Medication? Monitor Your Health
Our Core Health 45 blood test measures 45 biomarkers including liver function, HbA1c, thyroid hormones, vitamins, and kidney function — the essential markers your doctor needs to see while you are on GLP-1 medications.
View Core Health 45 Blood TestLiver Fat Reduction: Retatrutide's Biggest Advantage
Perhaps the most striking difference between retatrutide and its predecessors is the effect on liver fat. Metabolic dysfunction-associated steatotic liver disease (MASLD, formerly called NAFLD or fatty liver disease) affects roughly one in three adults in the UK and is closely linked to obesity, insulin resistance, and cardiovascular disease. Monitoring liver function markers is critical for anyone concerned about metabolic health.
Semaglutide produces modest reductions in liver fat, typically in the range of 15–20%. It can improve liver enzyme levels but has not been shown to consistently resolve fatty liver disease.
Tirzepatide has demonstrated more impressive results. The SYNERGY-NASH Phase 2 trial showed that 62% of participants on the highest dose (15 mg) achieved resolution of MASH (metabolic dysfunction-associated steatohepatitis) at 52 weeks. More than half showed improvement in liver fibrosis. These results were significant enough for Eli Lilly to pursue Phase 3 liver-specific trials.
Retatrutide has produced the most dramatic liver fat reduction ever seen in a clinical trial for any medication. In a Phase 2a study published in Nature Medicine, the 12 mg dose reduced liver fat by an average of 86% at 48 weeks. Even more remarkably, 93% of participants on the 12 mg dose achieved normal liver fat levels (below 5%). At 24 weeks, the 12 mg group already showed 82.4% liver fat reduction, with 86% achieving normalisation.
This effect is likely driven by retatrutide's glucagon receptor agonism. Glucagon promotes hepatic fat oxidation, essentially instructing the liver to burn its stored fat for energy. When combined with the appetite suppression and metabolic benefits of GLP-1 and GIP, the result is a comprehensive assault on liver fat from multiple angles simultaneously.
For the millions of people in the UK living with fatty liver disease, these results represent a potential step change in treatment. If you are concerned about your liver health, a comprehensive blood test measuring ALT, AST, GGT, and ALP can provide an early warning of liver stress.
Side Effects: How Do They Compare?
All three medications share a common side effect profile dominated by gastrointestinal symptoms, which is expected given their mechanism of action. However, the severity and specific patterns differ.
Gastrointestinal Effects
Nausea, vomiting, diarrhoea, and constipation are the most frequently reported side effects across all three drugs. These symptoms tend to be most pronounced during dose escalation and generally improve as the body adjusts over 4 to 8 weeks.
Semaglutide trials reported nausea in approximately 44% of participants, vomiting in 24%, and diarrhoea in 30%. Discontinuation due to side effects occurred in approximately 7% of participants.
Tirzepatide showed a somewhat better gastrointestinal profile in some trials, with nausea around 33% and vomiting in 12% at the highest dose. In the SYNERGY-NASH trial, 96% of GI adverse events were mild to moderate.
Retatrutide in the TRIUMPH-4 Phase 3 trial showed nausea in 38–43%, vomiting in 20–21%, and diarrhoea in 33–35% depending on dose. Discontinuation rates were higher at 12–18%, partly because some participants discontinued due to what was described as perceived excessive weight loss.
Unique Safety Signals
Each drug has at least one distinctive safety concern:
- Semaglutide: A rare but documented risk of pancreatitis, and a boxed warning regarding thyroid C-cell tumours based on animal studies (no confirmed human cases at standard doses). Regular thyroid monitoring is recommended.
- Tirzepatide: Higher rates of gallbladder-related events, including gallstones and cholecystitis, which may be related to the rapid rate of weight loss.
- Retatrutide: A new safety signal emerged in the Phase 3 TRIUMPH-4 trial: dysesthesia (abnormal sensations of touch, such as tingling, numbness, or a burning feeling). This occurred in 8.8% of the 9 mg group and 20.9% of the 12 mg group, compared to just 0.7% on placebo. Dysesthesia was not reported at significant rates in earlier Phase 2 trials. These events were generally mild and rarely led to treatment discontinuation, but this will be an important focus in ongoing safety monitoring.
For all three medications, monitoring cholesterol levels, liver enzymes, kidney function, and blood sugar is recommended at regular intervals during treatment.
Availability and Cost in the UK
Semaglutide (Wegovy / Ozempic)
Semaglutide is the most established option in the UK. Wegovy (the weight-management formulation) has been approved by the MHRA and is available on the NHS under strict criteria, though supply has been intermittent. Private prescriptions are widely available through online pharmacies and weight management clinics. Monthly costs range from approximately £130 to £295 depending on dose and provider, with the maintenance dose of 2.4 mg typically at the higher end.
Tirzepatide (Mounjaro)
Mounjaro received MHRA approval and is available in the UK, though NHS access remains extremely limited. Under the current phased rollout, NHS prescribing is prioritised for patients with a BMI of 40 or above and at least four weight-related health conditions. Privately, Mounjaro costs have risen significantly following Eli Lilly's September 2025 price increase, with monthly costs now ranging from approximately £149 to £375 depending on dose. At maintenance doses, Mounjaro is generally more expensive than Wegovy.
Retatrutide
Retatrutide is not available anywhere in the world outside of clinical trials. It remains an investigational drug. Eli Lilly's TRIUMPH Phase 3 programme includes multiple trials expected to complete throughout 2026. If results are positive, regulatory submissions to the FDA, EMA, and MHRA could follow in late 2026 or 2027. Realistically, UK commercial availability may not come until 2027 or 2028, depending on the speed of MHRA review and subsequent NICE appraisal. Pricing has not been announced but is expected to be in a similar or higher range than tirzepatide given its expanded mechanism.
Be cautious of any online sources claiming to sell retatrutide. Any product being sold as retatrutide before regulatory approval is unregulated, unverified, and potentially dangerous.
Which Medication Might Be Right for You?
The choice between these medications is not simply about picking the one with the highest weight loss percentage. Several factors should guide the discussion with your doctor.
Decision Framework
Consider semaglutide if:
- You have established cardiovascular disease (it is the only one with proven heart benefit from the SELECT trial showing a 20% reduction in major adverse cardiovascular events)
- You want the most extensively studied option with the longest real-world safety data
- Budget is a primary concern (generally the least expensive option)
- You need an oral option (Rybelsus, the tablet form of semaglutide, is available for type 2 diabetes)
Consider tirzepatide if:
- You need greater weight loss than semaglutide has provided (or your doctor anticipates you will)
- You have type 2 diabetes and need robust HbA1c reduction (up to 2.4%)
- You have MASLD/MASH and want liver-specific benefits
- You want an available, proven option that outperforms semaglutide
Retatrutide may be worth discussing with your doctor when available if:
- You have significant fatty liver disease (its liver fat reduction is unmatched)
- Previous weight loss medications have not achieved sufficient results
- You have complex metabolic syndrome requiring broad metabolic improvement
- You are willing to participate in or wait for clinical trials
Regardless of which medication you take or are considering, the single most important supporting action you can take is regular blood testing. Weight loss medications affect multiple organ systems, and monitoring ensures both safety and efficacy.
Blood Tests You Should Have with Any Weight Loss Medication
Whether you are on semaglutide, tirzepatide, or eventually retatrutide, regular blood monitoring is essential. These medications cause significant metabolic changes, and blood tests help your doctor ensure those changes are safe and heading in the right direction.
Here are the key biomarkers to monitor:
| Test Category | Key Biomarkers | Why It Matters |
|---|---|---|
| Blood Sugar | HbA1c, fasting glucose | Track diabetes risk reduction and medication efficacy |
| Liver Function | ALT, AST, GGT, ALP, bilirubin | Monitor for liver stress and track fatty liver improvement |
| Kidney Function | Creatinine, eGFR, urea | Rapid weight loss and reduced intake can affect kidney health |
| Thyroid | TSH, free T4, free T3 | GLP-1 drugs carry thyroid warnings; monitoring is recommended |
| Lipid Profile | Total cholesterol, HDL, LDL, triglycerides | These drugs improve cholesterol; testing confirms the benefit |
| Vitamins & Minerals | Vitamin D, B12, folate, iron, ferritin | Reduced food intake increases deficiency risk |
| Inflammation | CRP, ESR | Weight loss should reduce systemic inflammation |
| Pancreatic | Amylase, lipase | Monitor for pancreatitis risk (rare but serious) |
We recommend testing at baseline (before starting medication), at 3 months, and then every 6 months while on treatment. If your dose changes significantly or you experience side effects, additional testing may be warranted.
Our Core Health 45 blood test covers 45 biomarkers across all of these categories, giving you and your doctor a comprehensive view of how your body is responding to treatment. For the most detailed picture, including advanced inflammatory markers and a full thyroid panel, our Peak Insights 70 measures 70 biomarkers and is ideal for anyone on long-term weight management medication.
Whichever Medication You Choose, Monitor with Blood Data
Whether you opt for retatrutide, semaglutide, or tirzepatide, all three require monitoring of the same core biomarkers: HbA1c, liver enzymes, lipids, kidney function, and thyroid markers. A comprehensive blood test before starting and at regular intervals ensures safe, data-driven treatment regardless of which drug you use.
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Frequently Asked Questions
Is retatrutide better than semaglutide?
In terms of weight loss efficacy and liver fat reduction, retatrutide has produced superior results in clinical trials. However, semaglutide has far more real-world data, proven cardiovascular benefits from the SELECT trial, and is actually available for prescription today. "Better" depends entirely on your individual health needs, and retatrutide is not yet approved for use.
When will retatrutide be available in the UK?
Retatrutide is currently in Phase 3 clinical trials under the TRIUMPH programme. If results continue to be positive, Eli Lilly could submit for MHRA approval in late 2026 or 2027. Following regulatory review and NICE appraisal, UK availability could come in 2027 or 2028. This timeline is subject to change based on trial outcomes and regulatory processes.
Can I switch from semaglutide to tirzepatide?
Yes, many prescribers do facilitate switches from semaglutide to tirzepatide, particularly if weight loss has plateaued or HbA1c targets have not been met. The switch should always be managed by your prescribing doctor, who will determine the appropriate starting dose of tirzepatide and monitor your response. A blood test before and after switching is advisable.
How much weight can you lose on retatrutide?
In the Phase 3 TRIUMPH-4 clinical trial, participants on the 12 mg dose lost an average of 28.7% of their body weight (approximately 71.2 lbs or 32 kg) over 68 weeks. Individual results varied, and these figures represent averages from a trial population with obesity and knee osteoarthritis. Real-world results may differ.
Does tirzepatide beat semaglutide for weight loss?
Yes. The SURMOUNT-5 head-to-head trial published in the New England Journal of Medicine showed tirzepatide achieved 20.2% body weight loss versus 13.7% for semaglutide at 72 weeks, representing a 47% greater relative weight loss. However, semaglutide remains the only one with completed cardiovascular outcomes data showing a 20% reduction in heart attacks and strokes.
What blood tests should I get while on Ozempic, Wegovy, or Mounjaro?
At minimum, you should monitor HbA1c, fasting glucose, a full liver function panel (ALT, AST, GGT), kidney function (creatinine, eGFR), thyroid function (TSH), a lipid profile, and key vitamins (D, B12, folate, iron). Testing at baseline and every 3 to 6 months is recommended. A comprehensive test like the Lola Health Core Health 45 or Peak Insights 70 covers all of these in a single at-home blood draw.
What is the new side effect found with retatrutide?
The Phase 3 TRIUMPH-4 trial identified dysesthesia as a new safety signal. Dysesthesia is an abnormal sensation of touch, such as tingling, numbness, or burning. It occurred in up to 20.9% of participants on the highest dose, compared to 0.7% on placebo. These events were mostly mild and rarely led to stopping treatment, but they were not observed at similar rates in earlier trials. Further study is needed to understand this effect fully.
Is retatrutide good for fatty liver disease?
The early data is extraordinarily promising. In a Phase 2a trial, retatrutide reduced liver fat by an average of 86% at 48 weeks, with 93% of participants on the highest dose achieving normal liver fat levels. No other medication, including tirzepatide, has come close to these results. However, these findings need to be confirmed in larger Phase 3 trials specifically designed for MASLD/MASH, and the drug is not yet approved for any condition.
Get the Full Picture with 70 Biomarkers
Our Peak Insights 70 blood test is the most comprehensive at-home health check available. Monitor liver function, HbA1c, thyroid, cholesterol, vitamins, inflammation, and more — everything you need alongside weight loss medication.
View Peak Insights 70 Blood TestImportant Notice: Lola Health provides at-home blood testing services. We do not prescribe, supply, or endorse any medications, including semaglutide, tirzepatide, or retatrutide. The information in this article is based on published clinical trial data and is intended for educational purposes only.
Retatrutide is an investigational medication that has not been approved by the MHRA, FDA, EMA, or any other regulatory authority. Do not attempt to obtain retatrutide from unregulated sources. Always consult a qualified healthcare professional before making decisions about weight loss medications or any other treatment.
Article written and medically reviewed: February 2026. Information is current as of publication date and will be updated as new trial data becomes available.
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