What are peptides, and why are they so popular?

So, what are peptides? A peptide is a chain of amino acids joined by amide bonds ^[1], the links usually known as peptide bonds. The therapeutic peptides that dominate the literature are usually described as having molecular weights of 500 to 5000 Da ^[1]. A protein is built from the same amino acids and the same bonds, only longer, and so the question of what separates the two is partly a question of where someone chooses to draw a line.

One line is written into United States federal regulations. For the purposes of that rule, a protein is any alpha amino acid polymer with a specific, defined sequence that is greater than 40 amino acids in size ^[2]. Where two or more chains are associated with each other in the way they are in nature, the size is based on the total number of amino acids in those chains rather than on any one contiguous sequence ^[2]. In that regulator's terms, then, a chain of 40 amino acids or fewer is a peptide and a longer one is a protein.

The practical contrast is about structure. Natural peptides consist of chains of amino acids joined by amide bonds, but they lack the stability that secondary or tertiary structures give a folded protein ^[1]. Without that folded shape, the amide bonds can be hydrolysed or destroyed by enzymes in the body, which makes a natural peptide chemically and physically unstable, with a short half-life and fast elimination ^[1]. That one property, a short life in the body, explains a large share of what the rest of this guide describes: why the body uses peptides as brief signals, and why chemists have spent a century trying to lengthen them.

This guide treats peptides as a class of molecules. It describes what a peptide is, the ones the body already makes, how they came to be made in a laboratory, why so many have become approved medicines, and what limits them. It also says where an investigational research peptide sits against an approved medicine, because the two are often discussed as if they were the same thing and they are not.

Long before anyone synthesised one, the body was making peptides and using them as messengers. Insulin is the best known: a peptide of 51 amino acids ^[1]. Another is glucagon-like peptide-1, or GLP-1, an incretin hormone with important effects on glycaemic control and body weight regulation ^[3]. A peptide hormone of this kind is released from one tissue, reaches cells carrying a matching receptor, and changes what those cells do.

GLP-1 is released from enteroendocrine cells in the gut and controls meal-related glycaemic excursions by augmenting insulin and inhibiting glucagon secretion ^[4]. It also inhibits gastric emptying and food intake ^[4]. Drugs that act on it, the GLP-1 receptor agonists, mediate their effects through the GLP-1 receptor (GLP-1R), which is expressed in the pancreas, the gastrointestinal tract, the heart, the lungs, the kidneys and the brain ^[3]. Studies of liraglutide and semaglutide found that GLP-1 receptors in the pancreas and in the brain account for the improvements in glycaemic control and in body weight respectively ^[3].

The reviews disagree slightly on the size of the native hormone. One describes GLP-1 as a 30-amino-acid peptide hormone ^[3]; another calls it a 37-amino-acid peptide ^[1]. Both agree on the property that matters most for medicine: its half-life is very short ^[1], measured in human studies at 1.5 minutes after intravenous dosing and 1.5 hours after subcutaneous dosing ^[3].

Early peptide medicines followed this pattern closely. Peptide drug discovery began with endogenous human peptides ^[5], and the first efforts were focused on human hormones, before widening to peptides derived from nature and to structures found through medicinal chemistry ^[5], ^[6]. The shape of the field today, in which a short natural signal is redesigned to last longer, is the continuation of that first idea.

The history of peptide medicine starts with insulin. It was first isolated by Frederick Banting in 1921 and was available to people with diabetes mellitus a year later ^[1]. In 1923 it became the first commercial peptide drug ^[1]. A 2018 review dates peptide therapeutics as a notable part of medical practice from the advent of insulin therapy in the 1920s ^[5].

Making peptides on demand, rather than extracting them, depended on chemical synthesis. The chemical synthesis of peptides is well developed, particularly solid-phase peptide synthesis, which Merrifield developed in 1963 ^[1]. The method has been improved since 1963 and plays a crucial role in modern peptide production ^[1]. Peptides are now produced and modified by both chemical and biological methods, together with newer design and delivery strategies that have helped to overcome the inherent drawbacks of peptides ^[1].

The medicinal chemistry that followed is described in a 2021 Perspective in Nature Reviews Drug Discovery, which traces early efforts focused on human hormones, rational design, peptide drugs derived from nature, and breakthroughs in molecular biology and peptide chemistry ^[6]. It also names emerging strategies such as integrated venomics and peptide-display libraries ^[6]. The common thread is that chemists learned to make peptides, and then learned to change them.

The clearest case of changing one is GLP-1. Its discovery led to efforts to extend its half-life, and the development of short-acting and then long-acting GLP-1 receptor agonists followed ^[3]. For liraglutide and semaglutide, reversible binding to albumin was used to prolong action in the circulation ^[3]. A systematic test of fatty acid derivatives from C12 to C20 found that a C18 di-acid with a γGlu-2xOEG linker gave the highest albumin affinity combined with GLP-1 receptor potency ^[3]. More broadly, approved peptide drugs such as liraglutide, semaglutide and insulin degludec carry C14, C16 or C18 fatty acids, which increased their time in the plasma and reduced degradation during elimination by the kidney ^[1].

1. 1921

   Insulin, a peptide of 51 amino acids, is first isolated by Frederick Banting.[1]

2. 1923

   Insulin becomes the first commercial peptide drug.[1]

3. 1963

   Merrifield develops solid-phase peptide synthesis, now central to modern peptide production.[1]

4. After GLP-1's discovery

   Efforts to extend the half-life of GLP-1 lead to short-acting and then long-acting GLP-1 receptor agonists, among them liraglutide and then semaglutide.[3]

Four things explain most of the attention. The first is the number of medicines. Since the introduction of insulin almost a century ago, more than 80 peptide drugs have reached the market for a wide range of diseases, including diabetes, cancer, osteoporosis, multiple sclerosis, HIV infection and chronic pain ^[6]. A 2022 review gives the same figure of more than 80 approved worldwide ^[1]. An earlier 2018 review counted over 60 approved in the United States and other major markets and more than 150 peptides in active clinical development ^[5]. The two counts use different geographies, so they are not a clean growth rate, but both describe a class that kept enlarging.

The second is commercial scale. Peptide drugs account for a significant proportion of the pharmaceutical market, with worldwide sales that more than doubled between 2013 and 2019 ^[1]. That is a market observation reported by the review, and it says nothing about any single compound.

The third is engineering. Natural peptides have a short half-life and fast elimination ^[1], and the work described in the previous section is largely the story of making them last. Semaglutide shows the result: it has an approximate half-life of one week, helped by extensive albumin binding ^[9]. Compared with 1.5 minutes for native GLP-1 given intravenously ^[3], that is a change of several orders of magnitude, and it is what makes once-weekly administration possible in the trials below.

The fourth is the receptor. The GLP-1 receptor agonists mediate their effects via the GLP-1 receptor, which is expressed in the pancreas, the gastrointestinal tract, the heart, the lungs, the kidneys and the brain ^[3]. Significant weight loss led to the approval of liraglutide (3.0 mg) for the treatment of obesity, an indication under investigation with semaglutide at the time of that review ^[3].

Large trials of GLP-1 receptor agonists have also been widely reported. They are worth reading in their own terms, as two separate studies of different drugs.

Peptides are also being discussed outside the clinic. A 2026 review in Missouri Medicine notes that injectable peptides are increasingly used without prescriptions, driven by digital marketing and social media ^[12]. That is a description of a trend, and part of the context for the section on research peptides below.

Peptides have well-documented drawbacks, and the engineering described above exists because of them. The first is stability: peptides have poor stability in vivo, and the amide bonds can be hydrolysed by enzymes in the body, leaving a short half-life and fast elimination ^[1]. Fatty acid conjugation and albumin binding are one answer to it ^[1], ^[3].

The second is the route. Oral delivery is desirable, but acceptable bioavailability has not been achieved for large hydrophilic peptides and proteins, because oral absorption typically requires a low molecular weight and a certain hydrophobicity ^[3]. A 2026 review in Pharmaceutics states that typically less than 1% of orally administered peptide therapeutics reaches the systemic circulation ^[10]. For oral semaglutide, absolute bioavailability is only approximately 0.4 to 1% under optimised fasting conditions with restricted water intake, and systemic exposure is essentially absent in the fed state ^[9].

The reasons are physical. A 2026 review attributes the low exposure of oral semaglutide to peptide degradation within the gastrointestinal tract, poor epithelial permeability, gastric physiological constraints and presystemic metabolism ^[9]. The oral form uses an absorption enhancer, sodium N-(8-[2-hydroxybenzoyl]amino) caprylate (SNAC), which helps gastric absorption by raising the local pH, reducing acid-induced peptide unfolding and suppressing pepsin activation ^[9]. The Pharmaceutics review lists enzymatic degradation and pH effects, motility, microbiota, transporters, tight junctions and inflammation as the physiological factors that govern oral peptide absorption ^[10]. It adds that transporter-mediated uptake appears unlikely to account for the absorption of intact peptide drugs, because of their size ^[10].

The third limit is manufacturing. The 2025 analysis of FDA approvals notes that peptide manufacturing and sustainability concerns, including the use of PFAS, remain key challenges, and that the high cost of innovative therapies limits access, particularly in low- and middle-income countries ^[11]. The solid-phase method that made peptides practical has also been improved since 1963 ^[1], so the situation is not static. It is a field in which the main difficulties are known and are being worked on.

• Stability: amide bonds are broken by enzymes, so natural peptides have short half-lives ^[1].
• Half-life: native GLP-1 lasts 1.5 minutes after intravenous dosing and 1.5 hours after subcutaneous dosing in humans ^[3].
• Oral route: typically less than 1% of an orally administered peptide reaches the circulation ^[10]; oral semaglutide manages approximately 0.4 to 1% ^[9].
• Manufacture: peptide manufacturing and sustainability concerns remain key challenges ^[11].

An approved peptide medicine is one a regulator has reviewed and licensed for a defined use. In 2025, the FDA approved 44 new drugs, and one of them was a peptide ^[11]. Semaglutide and tirzepatide are described in a 2026 review as evidence-based, FDA-approved peptides that offer significant clinical benefits under medical supervision ^[12]. Insulin is the oldest example of the class ^[1].

An investigational compound is one still being tested. Retatrutide is an investigational triple agonist, not an approved medicine, and the same review lists it among agents that lack established human safety profiles ^[12]. Other compounds, including BPC-157, TB-500 and melanotan-II, are described by that review as unapproved peptides that lack established human safety profiles ^[12]. The review says that an expanding illicit market provides easy access to many unapproved experimental drugs without a prescription, and it lists product contamination, manufacturing impurities and inaccurate dosing among the potential risks ^[12].

This is where the term research peptide needs care. In this guide it means a compound supplied for laboratory investigation rather than as a medicine, and compounds sold as research reagents are not approved for human use. Such a compound may have a large human trial record, a small one, or none, and the status of a molecule in the table above says what a regulator or a reviewing physician has concluded, not what a laboratory supplier can claim. The trials in the previous section were run on specific products, in specific populations, under medical supervision. They are evidence about those products.

What remains open is considerable. The reviews describe stability, delivery and manufacture as problems that are being worked on, not solved ^[1], ^[9], ^[10], ^[11]. Oral peptide absorption in particular is still poorly understood in its mechanisms, with the Pharmaceutics review finding no single dominant pathway ^[10]. The accurate summary of the field is that a handful of peptides have been engineered into well-studied medicines, a larger number are in development, and the evidence for many others is thin or absent.