Peptides for Weight Loss: What Does the Evidence Show?

Peptides are a popular topic these days in the realm of weight management. Medicines like semaglutide and tirzepatide have revolutionized the management of obesity, and other experimental drugs like retatrutide and AOD-9604 have captured a lot of online attention.

This has led to a certain degree of confusion as well.

The words ‘peptides for weight loss’ are used as if they were all a single proven class of weight loss drugs. They do not. Certain peptide drugs have been thoroughly researched in clinical trials of a large patient sample and are approved for weight loss in the UK. Some are still in trial or do not have sufficient human trials, or are not approved as drugs for humans.

This is important to consider when examining the research, because they are different.

This guide is intended to clarify what peptides are, the effect that peptide-based medicines can have on appetite and body weight, existing treatments for weight loss, and why experimental weight loss peptides are not a substitute for licensed medicines.

What are peptides?

Peptides are short chains of amino acids, the same building blocks that make up proteins.

The distinction between a peptide and a protein is not always defined by a strict size threshold, although peptides are generally smaller than proteins. Despite their small size, many peptides perform important biological functions.

Others act as signalling molecules to enable the communication between cells and organs.

Peptide hormones play roles in:

• appetite and satiety
• digestion
• blood glucose regulation
• insulin secretion
• growth
• inflammation
• cardiovascular function
• reproduction
• energy metabolism

This is one of the reasons for the importance of peptides in drug research.

Some medicines are intended to mimic or alter signalling systems which occur naturally in the body.

Glucagon-like peptide-1 (GLP-1) is one such important example.

What does GLP-1 have to do with weight?

GLP-1 is a peptide hormone released primarily from the intestine in response to food.

It has several physiological effects after eating, including stimulating glucose-dependent insulin secretion, influencing gastrointestinal activity and contributing to signals involved in satiety.

Put simply, GLP-1 forms part of the signalling network between the gut, pancreas and brain.

This connection is also known as the gut-brain axis.

Researchers found that targeting the GLP-1 receptor pharmacologically could have useful metabolic effects. This led to the development of GLP-1 receptor agonists.

These medicines are designed to remain active for considerably longer than naturally occurring GLP-1.

They can cause:

• reduced appetite
• increased feelings of fullness
• slower gastric emptying
• improved glucose regulation
• lower overall energy intake

In eligible patients, clinical trials have shown that some GLP-1-based medicines can produce clinically meaningful weight loss when used alongside appropriate dietary and lifestyle measures.

However, not all peptides involved in metabolic research act through the GLP-1 receptor. Researchers are also studying medicines that target multiple metabolic receptors.

Do peptides work for weight loss?

Some do, but ‘peptides’ are far too broad a category for a simple yes-or-no answer.

Asking whether peptides work for weight loss is similar to asking whether medicines work for pain: the answer depends on the specific compound and the quality of evidence supporting it.

Some peptide-based medicines have demonstrated clinically meaningful effects on body weight in large randomised controlled trials. Semaglutide and tirzepatide are important examples.

Both are established prescription medicines with licensed indications in the UK.

That evidence shouldn’t be taken to mean that every compound sold online as a “weight-loss peptide” is the same.

A laboratory peptide is in a very different stage of development than a medicine that has been the subject of several completed phase 3 clinical trials and regulatory review.

Therefore, the quality and level of evidence is as important as the biological mechanism.

Which peptides are associated with weight loss?

There are many peptides that are commonly searched for when people are looking into weight loss. They vary considerably in their mechanisms, quality of clinical evidence and regulatory status.

Semaglutide

Semaglutide is a GLP-1 receptor agonist drug.

It is an analogue of naturally occurring GLP-1 that has been modified to remain active in the body for considerably longer.

By influencing appetite and satiety, semaglutide can reduce energy intake when used as part of an appropriate weight-management programme.

In the UK, semaglutide is available under different brand names with different licensed indications.

Wegovy is licensed for weight management in eligible patients when used according to its authorised indication.

Ozempic also contains semaglutide but is licensed primarily for the treatment of type 2 diabetes. Medicines containing the same active ingredient do not necessarily have the same licensed indication.

Semaglutide is a prescription drug.

Tirzepatide

Tirzepatide is a newer peptide-based medicine used in metabolic disease.

Unlike semaglutide, which acts at the GLP-1 receptor, tirzepatide is a dual GIP and GLP-1 receptor agonist.

GIP is the abbreviation of glucose-dependent insulinotropic polypeptide. It is also an incretin hormone like GLP-1 that is involved in metabolic signalling after eating food.

Tirzepatide acts on both pathways, which affect energy intake and glucose regulation as well as appetite.

In the UK, tirzepatide is marketed as Mounjaro and has licensed indications that include type 2 diabetes and weight management in eligible adults.

Tirzepatide is a prescription-only medicine and should be used under the supervision of an appropriately qualified healthcare professional.

Liraglutide

Another GLP-1 receptor agonist is liraglutide.

It has been widely studied in diabetes and obesity and predates semaglutide and tirzepatide.

The effects of liraglutide on appetite and satiety are similar to other GLP-1 receptor agonists.

While the newer therapies have garnered more attention, liraglutide is still significant historically as a milestone in the development of GLP-1 receptor activation as a treatment option for obesity.

Retatrutide

Retatrutide is an investigational medicine currently being studied in clinical development.

Unlike semaglutide, which activates just GLP-1 receptors, or tirzepatide, which activates both GIP and GLP-1 receptors, retatrutide has been engineered to activate three different receptor systems:

GIP, GLP-1 and glucagon receptors.

It is therefore commonly described as a triple receptor agonist.

Clinical studies have reported substantial reductions in body weight, generating considerable scientific and public interest.

But an important difference has to be made:

Retatrutide is an investigational medicine, not an established licensed weight-loss treatment in the UK.

The results of the research should therefore not be interpreted as evidence of the approval of products that are promoted online as medicines.

Clinical trials involve investigational products that are precisely formulated and studied in a controlled environment. This is very different from buying an unlicensed product from an online vendor or social-media account.

AOD-9604

Another compound frequently discussed in connection with ‘fat-loss peptides’ is AOD-9604. It was synthesized from a portion of human growth hormone, and is used in research to examine its effects on lipid metabolism.

Early laboratory and animal studies generated research interest in whether some metabolic effects associated with growth hormone could be separated from its broader biological activity.

The clinical evidence for efficacy of AOD-9604 in treating obesity, however, is significantly less than that of licensed products like semaglutide and tirzepatide.

AOD-9604 isn’t a registered weight loss drug in the UK.

Claims that AOD-9604 is a proven ‘fat-burning peptide’ are not supported by sufficiently robust clinical evidence.

Licensed medicines and research peptides are not the same thing

This is one of the most important distinctions for anyone researching peptides for weight loss.

The existence of biological research into a peptide does not establish that it is safe or effective as a treatment.

Similarly, a product being labelled “research use only” does not represent regulatory approval for human use.

What are “fat-burning peptides”?

Fat-burning peptide’ is not a recognised medical classification; it is primarily a marketing term.

Maintaining a healthy level of body fat is much more difficult than turning on and off fat burning.

The appetite, energy intake, physical activity, gastrointestinal signalling, hormones, genetics and sleep, as well as a multitude of environmental factors, can affect body weight.

Experimental peptides can have different effects on individual components of these systems.

For instance, in a laboratory experiment a compound may affect growth-hormone signalling or lipid metabolism. This does not prove that the same product injected results in a clinically significant and safe reduction of fat in humans.

Therefore, words like “melts fat”, “targets belly fat” or “rapid fat-burning peptide” should be taken with a pinch of salt, unless backed by good human clinical studies.

Peptides and appetite regulation

Appetite is regulated by a complex network of hormonal and neural signals, which is one reason peptide hormones have become important targets in obesity research.

Willpower is not the only factor in eating behaviour.

Before, during and after meals, many hormones are secreted by the gastrointestinal tract.

These include GLP-1, GIP, peptide YY (PYY) and ghrelin.

These signals are combined by the brain with information on stored energy and other physiological signals such as those related to blood glucose.

Ghrelin is closely related to appetite signals, whereas hormones like GLP-1 and peptide YY induce satiety.

Medicines that are used today to treat obesity can take advantage of some elements of this signalling network.

For instance, GLP-1 receptor agonists increase signals of fullness and decrease signals of hunger. As a result, some patients experience reduced hunger and lower energy intake during treatment.

This is not about the simple concept that the pill will “burn fat” in your body.

Are peptide weight-loss injections safe?

The safety of any compound is solely dependent on the compound being discussed.

Prescribed drugs are tested in a controlled clinical trial and evaluated by the regulatory authorities. Even at that time, they have side effects and are not a good choice for all people.

The most frequent side effects of GLP-1 class weight-loss drugs typically involve gastrointestinal issues, and can include:

• nausea
• vomiting
• diarrhoea
• constipation
• abdominal discomfort
• reduced appetite

Medical assessment and continuous monitoring is important because there are less common, but potentially serious, complications.

Existing medical conditions, other medicines, pregnancy or plans for pregnancy and other individual risk factors are also factors a healthcare professional should take into account.

When it comes to safety with unlicensed or experimental peptides, the situation is far more complex.

Whereas, for many research compounds, there may be limited information regarding long-term effects in humans.

An extra risk with products that are acquired outside of regulated pharmaceutical supply chains is that the product’s contents may not accurately match the label.

The problem with unlicensed weight-loss peptides

The rise in the use of weight loss shots has created a market for weight loss injection products that are sold via the internet, social media, and messaging apps.

This poses a number of risks.

Unregulated products may contain an incorrect concentration, a different active ingredient, contaminants or an ineffective amount of the stated ingredient.

The Medicines and Healthcare products Regulatory Agency (MHRA) has issued repeated alerts to consumers on illegally sold medicines for weight loss.

The Medicines and Healthcare products Regulatory Agency (MHRA) has repeatedly warned consumers about illegally supplied and falsified weight-loss medicines, including products promoted through websites and social-media platforms.

This is an important difference between clinical research and the non-regulated peptide market.

Investigational medicines used in clinical research are not equivalent to unlicensed products offered for sale online.

Are peptides for weight loss legal in the UK?

Not all peptides can be legally classified under one category.

Some peptide products are prescription drugs. Others are experimental drugs, chemicals for research or off-label drugs.

Established prescription drugs like semaglutide and tirzepatide should only be obtained with a suitable prescription.

Judgement should be made by a clinician whether treatment is medically appropriate.

Qualifications may include medical history related to weight, BMI, past weight management procedures, or the course of treatment.

A medicine being licensed for weight management does not necessarily mean it is available to every eligible patient through the NHS; NHS access is determined separately according to clinical guidance and eligibility criteria.

Experimental peptides should not be considered an alternative to licensed weight-management medications.

What about “research-grade” peptides?

Research-grade peptides can be confused with what is commonly called a peptide for research.

Chemicals being supplied for laboratory work does not imply that they have been approved as medication.

Research-use products can be products that are intended for experiments like:

• receptor studies
• analytical chemistry
• cell-culture experiments
• biochemical assays
• preclinical research

These applications are quite different from giving a compound to an individual.

Clinical safety is not indicated by a statement of laboratory tested or high purity or research grade.

Chemical purity is just one aspect of pharmaceutical quality.

Medicines for injection must be carefully controlled during manufacture, be sterile, formulated, stable, dose controlled and be subject to strict packaging and regulatory controls.

Can you get peptides for weight loss through the NHS?

Some medicines for weight loss can be obtained via the NHS for individuals who fulfil specific clinical requirements.

Currently, the NHS has a pathway in place for medications like semaglutide and tirzepatide to be used in the treatment of obesity.

But not all weight management medications are accessible to anyone who wants to lose weight.

It is based on clinical factors and advice from NHS.

Weight-management medications are typically not used as a replacement for nutrition/behavioural/physical activity interventions, but rather in conjunction with them.

Anyone wanting medical advice on obesity should speak to their GP or an appropriate weight management service.

Are weight-loss peptides the same as GLP-1 medicines?

Not exactly.

The GLP-1 medicines are a subset of a larger class of peptides.

The main mechanism of action of semaglutide is via the GLP-1 receptor.

Tirzepatide works on both GLP-1 and GIP receptors.

Retatrutide is under study for activity on the GLP-1, GIP and glucagon receptors.

Other peptides that are referenced here in regards to weight loss can work in entirely new ways.

The use of the terms “GLP-1” and “peptide” interchangeably will cause confusion.

Which peptide is best for weight loss?

The answer to this question is no there is no scientifically responsible answer that tells you which peptide is the “best”.

Treatment is based on:

• If the medicine is licensed
• The quality of the clinical evidence.
• individual medical history
• BMI and weight related conditions.
• other medications
• contraindications
• side-effect profile
• previous treatment response

If an individual is eligible to be treated for obesity with a medication, a clinician can determine which available licensed obesity medications might be appropriate.

Do not use experimental peptides as a substitute for known peptides just because initial studies seem to show promise.

What does the future of peptide weight-loss research look like?

Progress is rapidly being made in the field.

With the effectiveness of GLP-1 receptor agonists, researchers have begun to explore the combined effect of several hormonal pathways instead of just one receptor.

Tirzepatide offered proof of the potential of dual GIP/GLP-1 receptor activation.

Retatrutide is developing a triple-agonist therapy targeting the GIP, GLP-1 and glucagon receptors.

Other combinations that include amylin, glucagon and other metabolic pathways are current areas of investigation.

Future treatments may thus be very different from current GLP-1 medications.

Promising clinical trial results and regulatory approval are two different things, however.

Efficacy and safety, manufacturing quality, overall benefit-risk balance must be shown to be acceptable before new treatments become established medicines.

Conclusion

The study of peptides has revolutionized the scientific knowledge and treatment of obesity.

Semaglutide and tirzepatide are clear examples of the potential of targeting peptide-hormone signalling to achieve clinically meaningful weight loss in a subpopulation of patients who are selected to benefit from this approach.

The effectiveness of these medications has sparked a far more popular online trade of so-called weight reduction peptides, though.

It is incorrect to “ group together” those products.

The difference between an experimental peptide, based mainly on lab, animal or preliminary human studies, and a large controlled clinical trial is substantial.

Retatrutide is an example of the potential of the next generation of metabolic medicines, and compounds like AOD-9604 provide an example of why biological plausibility is not enough to make an effective medicine.

Whereas the most beneficial questions to ask when evaluating any peptide for weight management are not just “Does this peptide burn fat?” but:

What human evidence supports it? Has it been adequately studied for safety? Is it licensed for this purpose? And what do UK regulators and clinical guidelines say about its use?

Those distinctions provide a much clearer picture of the science behind peptides and weight management.