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What Do We Mean When We Talk About Clinical Trials?
What Do We Mean When We Talk About Clinical Trials?
Dr. Zoë Lees (PhD)
Dr. Sophie Dix (BSc, PhD)
1 April 2026 | Updated: 4 August 2026
4 August 2026
Article Summary:
What is a clinical trial?
Why are clinical trials important?
Who takes part in clinical trials?
What’s measured in a clinical trial?
What are the STEP and SURMOUNT studies?
What did these studies show?
What does ‘on average’ mean?
A simple example of what these percentages mean
Does this mean everyone gets the same results?
Why do we talk about these clinical studies during your care?
References
You may see the phrase ‘clinical trials’ when reading about prescription weight loss medications like Wegovy or Mounjaro, or study names like STEP or SURMOUNT.
In this guide, we’ll explain what that means in simple terms and why it’s important.
Article Summary:
- Clinical trials are studies that test how medications work, and how safe they are.
- Before a medication can be used, the company that makes it must prove to medical regulators that it’s safe and that it works as claimed. Clinical trials are how they do this.
- In large clinical trials, people lost an average of 20.7% of their body weight with Wegovy, and 22.5% with Mounjaro in 72 weeks (1 year, 5 months).
- These results show what’s typical for many people, but your own results may be different.
What is a clinical trial?
A clinical trial is a study that tests how well a medication works, and how safe it is when taken by people with the health concerns it’s made for.
Before a medication can be approved for use, the company that makes it must prove with clear evidence that it’s safe and that it does what they claim it does. Clinical trials are how that evidence is collected. This is then reviewed by medical regulators, who decide whether a medication is safe and effective enough to be used by the public.
This evidence also helps doctors understand how to prescribe the medication, and how best to support their patients.
Why are clinical trials important?
Clinical trials exist because a company can’t simply say that a medication works. They have to prove it. The evidence must be strong enough to satisfy independent medical regulators before the medication is approved for use.
Clinical trials for weight loss medications answer questions such as:
- Is the medication safe?
- Does the medication actually work, and how well?
- Who is it suitable for, and who shouldn’t take it?
- How much weight do people lose on average?
- How long does weight loss take?
- What are the common side effects?
This information helps regulators decide whether to approve the medication, and helps doctors explain treatment options clearly and honestly.
Who takes part in clinical trials?
People who take part in clinical trials are adults with similar health needs to those who may use the medication once it’s approved. Medical teams monitor them closely and check their progress regularly.
Many thousands of people can take part in one group of studies.
What’s measured in a clinical trial?
In clinical trials for weight loss medication, doctors and scientists usually look at:
- Changes in body weight over time
- Health measures, such as blood sugar levels
- Side effects
Everyone is tracked using the same rules, so results can be compared fairly.
What are the STEP and SURMOUNT studies?
The names STEP and SURMOUNT are simply labels for groups of studies. They’re not treatments, but rather names used by scientists to help organise and reference the research.
The STEP and STEP UP studies looked at weight loss with Wegovy.
The SURMOUNT studies looked at weight loss with Mounjaro.
What did these studies show?
In the STEP UP study, people taking Wegovy lost an average of 20.7% of their body weight. [1]
In the SURMOUNT-1 study, people taking Mounjaro lost an average of 22.5% of their body weight. [2]
These results were seen after 72 weeks (1 year and 5 months), in people who were taking the medication once a week alongside regular exercise and a reduced-calorie diet.
What does ‘on average’ mean?
The weight loss figures show the typical result across all people in the study. Some people lost more weight, and some lost less. They’re not a prediction of what will happen for any one individual, because everyone’s different.
A simple example of what these percentages mean
A 22.5% average weight loss means that someone who weighs 100kg may lose an average of 22.5kg in 72 weeks (1 year, 5 months).
Remember, this is only an example to help explain the numbers. It’s not a promise or prediction of individual results.
Does this mean everyone gets the same results?
No. Clinical trials show what usually happened when the medication was tested in a large group of people, but everyone’s body is different.
Lots of factors can affect your results, such as your health, your lifestyle, how your body responds to the medication, how long you stay on treatment, and how closely the medication is taken as recommended.
This is why regular check-ins with your MedExpress clinical team are important.
Why do we talk about these clinical studies during your care?
We use clinical trial results to set realistic expectations and explain what’s typical, not guaranteed. They’re how we know it’s been tested, reviewed, and approved as safe and effective. Giving us the confidence to prescribe it, and you the reassurance that the medication you’re taking has been proven to work.
Find out what treatment is suitable for you with MedExpress.
References
- Wharton S, Freitas P, Hjelmesæth J, Kabisch M, Kandler K, Lingvay I, Quiroga M, Rosenstock J, Garvey WT; STEP UP trial group. Once-weekly semaglutide 7·2 mg in adults with obesity (STEP UP): a randomised, controlled, phase 3b trial. Lancet Diabetes Endocrinol. 2025 Nov;13(11):949-963. doi: 10.1016/S2213-8587(25)00226-8. Epub 2025 Sep 14. PMID: 40961952.
- Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, Kiyosue A, Zhang S, Liu B, Bunck MC, Stefanski A; SURMOUNT-1 Investigators. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022 Jul 21;387(3):205-216. doi: 10.1056/NEJMoa2206038. Epub 2022 Jun 4. PMID: 35658024.
Next scheduled review date: 4 August 2029
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Authors
Written by: Dr. Zoë Lees (PhD)
Dr. Zoë Lees is a medical writer at MedExpress specialising in obesity and health communication. She completed her PhD at the University of Glasgow, where she investigated adipose tissue (fat tissue) function in pregnancies complicated by pre-eclampsia and gestational diabetes. Her work focuses on translating complex clinical evidence into clear, engaging and evidence-based health information for patients and the public. At MedExpress, Zoë contributes to clinical content, educational resources and media communications across areas including weight management, GLP-1 therapies and metabolic disease. Zoë is particularly interested in improving public understanding of obesity as a chronic condition and ensuring patients have access to accurate, accessible health information grounded in current scientific evidence.
Medically reviewed by: Dr. Sophie Dix (BSc, PhD)
Dr. Sophie Dix is Head of Medical Affairs at MedExpress. A behavioural neuroscientist by training, Sophie has spent 30 years working at the intersection of medical research, pharmacology, technology and patient communication across the pharmaceutical industry, health technology and charity sectors. At MedExpress, Sophie's role is to ensure that everything the organisation does is grounded in the most current science. That means translating complex clinical evidence into accessible health information across the website, clinical pathways and media communications. It also means working with external press and broadcast media as a named expert commentator, making the science behind weight management and GLP-1 treatments understandable to a general audience. A core part of Sophie's work is building MedExpress's Real-World Evidence (RWE) programme: learning systematically from patient outcomes to understand what works in practice, not just in clinical trials. This patient-level insight feeds directly into improving patient experience and health outcomes. Sophie believes that patients make better decisions when they have access to accurate, evidence-based information. As an advocate for meaningful change in how obesity is understood, Sophie works to reduce stigma and raise awareness of the condition as an illness. This includes reframing the conversation around GLP-1s from aesthetics to public health, while highlighting the health benefits of weight loss and weight management.
Note from the experts
Remember: This blog shouldn’t be regarded as medical advice, diagnosis, or treatment. We make sure everything we publish is fact checked by clinical experts and regularly reviewed, but it may not always reflect the most recent health guidelines. Always speak to your doctor about any health concerns you have.