Why Am I Gaining Weight on Retatrutide?

Why am I gaining weight on retatrutide is a question that may arise for participants enrolled in a clinical trial of this investigational medicine. Retatrutide holds no marketing authorisation in the UK and is available only within authorised research settings. Weight gain during a trial can result from many factors, including dose titration, concurrent medications, underlying health conditions, or fluid shifts, rather than the medicine itself. This article outlines what the published trial evidence reports, what factors may influence individual weight changes, and when to seek medical advice from your trial team.

What is retatrutide and what is its regulatory status in the UK?

Retatrutide is an unlicensed investigational medicine available only within authorised clinical trials in the UK; it is not accessible through the NHS, private prescribers, or any pharmacy.

Retatrutide holds no marketing authorisation in the UK or anywhere else, meaning it can be used only within an authorised clinical trial.^[1][2] It is an investigational medicine being studied as a potential treatment for obesity and related metabolic conditions. Retatrutide works by activating three hormone receptors simultaneously: glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon receptors.^[9][10] This triple-receptor mechanism distinguishes it pharmacologically from currently licensed GLP-1 receptor agonists, though no authorised head-to-head comparisons exist.

Because retatrutide is unlicensed, it is not available through the NHS, through private prescribers, or through any pharmacy outside a formal research setting. Ongoing trials can be found on ClinicalTrials.gov by searching for retatrutide. If you believe you are taking retatrutide outside a clinical trial, any product sold online under this name is not a licensed medicine and may be falsified or unsafe. The MHRA has published guidance on the risks of buying medicines online from unregistered sources.^[3][4] Licensed weight-management treatments do exist and can be discussed with your GP if you are looking for an authorised option.

For anyone currently enrolled in a retatrutide trial, all questions about your treatment, including unexpected weight changes, should be directed to your trial team in the first instance. The trial team operates under a study protocol that governs how adverse events and unexpected findings are managed and reported.

Why might someone gain weight during a retatrutide clinical trial?

Weight gain during a retatrutide trial is often unrelated to the medicine and may reflect dose titration, fluid retention, concurrent medications, or underlying conditions such as hypothyroidism or PCOS.

Weight gain during a retatrutide clinical trial can occur for several reasons that are unrelated to the medicine itself, and identifying the cause requires careful clinical assessment by the trial team. Trials enrol participants at a specific point in time, and individual weight trajectories vary considerably depending on factors such as dietary intake, physical activity, sleep quality, stress, and concurrent health conditions.

Dose titration is a particularly relevant factor. Retatrutide, like other medicines in its class, is introduced at a low starting dose and increased gradually over a number of weeks. During the early titration phase, the appetite-suppressing effects may not yet be fully established, and some participants may not notice a meaningful change in hunger or food intake at lower doses. This can mean that weight remains stable or may increase slightly before the therapeutic dose is reached.

Other considerations include:

• Fluid retention, which may temporarily increase body weight independently of fat mass

• Changes in muscle mass, particularly if physical activity levels have changed during the trial period

• Concurrent medications, some of which are associated with weight gain, including certain antidepressants, corticosteroids, antipsychotics, and insulin. Do not alter any concomitant medicines without first discussing this with your trial team

• Underlying medical conditions such as hypothyroidism, polycystic ovary syndrome (PCOS), or Cushing's syndrome, which can drive weight gain regardless of treatment

It is also worth noting that clinical trials measure outcomes across populations, and individual responses within a trial can differ substantially from the group average. Weight gain in one participant does not indicate that the medicine is ineffective overall, nor does it mean the medicine is causing harm.

What did the retatrutide trials actually report about weight changes?

A Phase 2 trial reported a mean body weight reduction of approximately 24% at 48 weeks at the highest dose studied, though individual responses within trials vary and some participants may not lose weight.

The most detailed published evidence on retatrutide and body weight comes from a Phase 2 trial reported in the New England Journal of Medicine in 2023 (Jastreboff et al.), which enrolled adults with obesity over 48 weeks. In that trial, participants receiving the highest dose studied (12 mg) lost a mean of approximately 24% of their body weight compared with placebo at 48 weeks.^[5][6] This figure relates to a specific dose, time point, and analysis population, and should not be read as a prediction of what any individual participant would experience.

Phase 3 trial programmes, including TRIUMPH and TRANSCEND, have since reported further data in larger and more diverse populations, including people with type 2 diabetes.^[7][8] Results from TRANSCEND-T2D-1 have been published in The Lancet.^[7][8] These findings extend the evidence base but remain subject to ongoing regulatory evaluation.

Within any trial, there is a distribution of responses: some participants lose substantially more than the average, some lose less, and a small number may not lose weight or may gain weight during the study period. Trials are designed to detect effects at a population level, not to guarantee a uniform outcome for every individual.

It is also important to understand that trial participants are selected according to strict eligibility criteria, follow structured dietary and lifestyle guidance, and are monitored closely throughout. These conditions differ from everyday clinical practice, which means trial results may not translate directly to real-world settings even if the medicine were eventually authorised. The evidence base continues to develop, and regulatory agencies including the MHRA will evaluate the totality of trial data before making any authorisation decision.

Could side effects or other factors be affecting your weight?

Gastrointestinal side effects, reduced physical activity during titration, hormonal factors, and psychological responses to trial participation can all plausibly influence weight independently of the medicine's primary action.

Several factors associated with participation in a clinical trial, or with the medicine's known side effect profile, could plausibly influence weight in ways that are not straightforward. The gastrointestinal side effects reported in retatrutide trials, including nausea, vomiting, and diarrhoea, are common during dose escalation.^[5][6] While these effects often reduce appetite and contribute to weight loss in many participants, they can also lead to dehydration and subsequent fluid shifts once symptoms settle, which may temporarily affect the number on the scales. If gastrointestinal symptoms are persistent or severe, contact your trial team rather than managing them alone.

If gastrointestinal side effects disrupt normal eating patterns significantly, some participants may find they consume more easily tolerated, calorie-dense foods once symptoms improve. This is a plausible response observed with medicines that cause nausea during titration, though it has not been formally quantified in retatrutide-specific data.

Other factors worth considering include:

• Reduced physical activity due to fatigue or nausea during the titration phase, which may reduce energy expenditure

• Psychological responses to trial participation, including anxiety or changes in sleep, both of which are associated with weight regulation

• Hormonal factors, particularly relevant for women, as menstrual cycle phase, perimenopause, and hormonal contraception can all influence weight and fluid balance

Participants in retatrutide trials who are of childbearing potential should be aware that the trial protocol will specify contraception requirements, as pregnancy is an exclusion criterion. If you have any concerns about contraception, breastfeeding, or pregnancy during trial participation, raise these directly with your trial team, who can advise according to the study protocol.

When should you speak to a doctor about unexpected weight changes?

Unexpected weight gain during a retatrutide trial should be reported to the trial team first; rapid gain with swelling, breathlessness, or severe abdominal pain requires prompt medical assessment.

If you are enrolled in a retatrutide clinical trial and experiencing unexpected weight gain, your first point of contact should always be your trial team, who are best placed to assess whether the change is clinically significant and whether it requires any adjustment to your participation. Trial protocols include defined procedures for monitoring and responding to unexpected findings, and your trial team can access your full medical history and trial data in a way that a GP or other clinician outside the study cannot.

However, there are circumstances in which you should seek medical attention more promptly:

• Rapid, unexplained weight gain over a short period, particularly if accompanied by swelling of the legs or ankles, breathlessness, or changes in urine output, may indicate fluid retention related to a cardiac or renal condition and warrants prompt assessment

• Persistent abdominal pain, especially if severe or accompanied by vomiting, should be assessed urgently, as acute pancreatitis has been reported in association with medicines in this class, including post-marketing reports of necrotising pancreatitis^[17]

• Symptoms suggesting a thyroid or adrenal problem, such as persistent fatigue, cold intolerance, or significant changes in mood or energy, should be discussed with a doctor

Whenever you attend urgent or emergency care, tell the clinician that you are participating in a clinical trial and show them your trial ID card or study contact details. This helps them access relevant information about your treatment and report any findings through the correct channels.

For urgent advice that does not require an emergency response, contact NHS 111. If you experience a life-threatening emergency, call 999 or go to your nearest accident and emergency department. Suspected side effects from medicines taken within a clinical trial should be reported to your trial team under the study protocol; they may also be reported to the MHRA via the Yellow Card scheme at yellowcard.^[18][19]mhra.gov.uk.

Scientific References

1. Retatrutide for managing overweight and obesity [ID6644].
2. EMA Paediatric Investigation Plan Decision – Retatrutide (EMEA-003258-PIP02-23).
3. MHRA urges public to avoid illegal online weight-loss medicines this new year.
4. Medicines information.
5. Triple–Hormone-Receptor Agonist Retatrutide for Obesity (Jastreboff et al.).
6. Triple-Hormone-Receptor Agonist Retatrutide for Obesity (Jastreboff et al.) – PubMed.
7. Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist – TRANSCEND-T2D-1 (Bajaj et al.).
8. Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist – TRANSCEND-T2D-1 (Bajaj et al.) – PubMed.
9. Retatrutide showing promise in obesity (and type 2 diabetes). Expert Opinion on Investigational Drugs.
10. Cardiovascular effects of incretin receptor agonists: beyond glycemic control. Canadian Journal of Physiology and Pharmacology.
11. Incretin-based therapies in diabetic kidney disease: toward integrated cardio-kidney-metabolic disease modification. Kidney Research and Clinical Practice.
12. Advances in the management of obesity: non-pharmacological strategies, herbal formulations, current pharmacotherapy, emerging drugs, and future perspectives. Frontiers in Endocrinology.
13. Efficacy and Safety of Dual and Triple GLP-1-Based Polyagonists in MASLD/MASH: A Systematic Review and Meta-Analysis. Cureus.
14. The promise of GLP-1 receptor agonists for the treatment of obesity: a look at phase 2 and 3 pipelines. Expert Opinion on Investigational Drugs.
15. Drugs Affecting Body Weight, Body Fat Distribution, and Metabolic Function – Mechanisms and Possible Therapeutic or Preventive Measures: an Update. Current Obesity Reports.
16. Bodyweight change as an adverse effect of drug treatment: mechanisms and management. Drug Safety.
17. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis including necrotising and fatal cases.
18. The Yellow Card scheme: guidance for healthcare professionals, patients and the public.
19. Adverse reactions to drugs | Medicines guidance – BNF – NICE.

Frequently Asked Questions