What Are Peptides? How They Work in the Body - and Which Ones Are Prescribed

Key takeaways

A peptide is a short chain of amino acids, conventionally 2 to 50, joined by peptide bonds. Many of the body's own signals are peptides: insulin, GLP-1, oxytocin, growth hormone-releasing hormone.
Peptide drugs work by copying those signals, usually modified so they survive in the blood long enough to act. Most are injected because the gut digests them.
"Peptides" in 2026 means four different things: FDA-approved drugs, compounded drugs made from permitted ingredients, unregulated vials sold as "research" products, and cosmetic or food peptides. The legal status, quality control and evidence differ completely.
The evidence ranges from the strongest in obesity medicine (GLP-1 drugs: about 15 to 21% weight loss in trials) to zero human trials (TB-500, by FDA's own account in 2026). Most peptides marketed online sit near the weak end.
The only way to get a peptide with a known content and dose is a prescription from a licensed clinician filled by a licensed pharmacy or a brand manufacturer.

What a peptide is

Proteins and peptides are made of the same twenty amino acids, linked end to end by a chemical bond called the peptide bond. The difference is length. By convention, a chain of roughly 2 to 50 amino acids is a peptide; longer chains that fold into complex three-dimensional machines are proteins. Insulin, with 51 amino acids in two linked chains, sits right at the border and is usually called a peptide hormone.

The body makes thousands of peptides, and most of them are messages. A cell in the gut releases GLP-1 after a meal; it travels to the pancreas and brain and says "food is here, release insulin, slow the stomach, stop eating." The hypothalamus releases growth hormone-releasing hormone (GHRH) in pulses at night; it travels a few millimetres to the pituitary and says "release growth hormone now." Oxytocin, vasopressin, calcitonin, glucagon, the natriuretic peptides that regulate blood pressure, the endorphins: all peptides, all signals.

Two properties follow from the chemistry, and they explain almost everything about peptide drugs:

Peptides are specific. A peptide fits a particular receptor on the surface of particular cells, so it tends to do one job with few off-target effects. This is the main attraction of peptides as drugs; reviews in Nature Reviews Drug Discovery and elsewhere describe their high potency and selectivity compared with small-molecule drugs.
Peptides are fragile. Enzymes in the gut, blood and tissues cut them apart in minutes to hours, and the stomach digests a swallowed peptide like any other protein. That is why most peptide drugs are injected, and why the natural versions almost never make useful medicines without modification.

How peptide drugs work

A peptide drug is a copy of a natural signal, engineered to last. Semaglutide is human GLP-1 with two amino acids swapped to block the enzyme that destroys GLP-1, plus a fatty-acid chain that makes it bind to albumin in the blood; the result is a signal the body would normally clear in two minutes that now lasts a week. Tirzepatide is a single peptide built to activate two receptors at once (GLP-1 and GIP). Sermorelin is the first 29 amino acids of GHRH, which is the part that does the work. Tesamorelin is a GHRH analog with a chemical cap that slows its breakdown.

Once injected, the peptide binds its receptor and the cell does what it would do in response to the natural signal: release insulin, suppress appetite, release growth hormone, contract smooth muscle, signal satiety. The drug does not add a new ability to the body. It turns up a signal the body already uses, for longer and at a higher level than it would on its own. That framing predicts both the benefits and the side effects: a GLP-1 drug produces the nausea and slowed stomach of an intense post-meal signal; a GHRH analog produces the fluid retention and joint aches of raised growth hormone.

A short history

Insulin, purified from animal pancreases in 1921 and 1922, was the first peptide drug and is still the most important. Synthetic chemistry produced oxytocin and vasopressin in the 1950s, and solid-phase synthesis in the 1960s made it practical to build any sequence. The 2021 review by Muttenthaler and colleagues counts more than 80 peptide drugs approved worldwide, with roughly 150 in clinical trials and several hundred in preclinical development; the 2022 review by Wang and colleagues reaches a similar count. Growth has accelerated since, driven almost entirely by the GLP-1 class, which by 2025 had become among the best-selling drugs in the world.

The four things people mean by "peptides"

The word covers products with nothing in common except their chemistry. Sorting them out is the single most useful thing a patient can do before spending money.

Category	What it is	Who makes it	Quality control	Examples
FDA-approved peptide drugs	Medicines that completed clinical trials and FDA review for a specific use	Pharmaceutical manufacturers	Full FDA oversight of manufacturing, labeling and safety reporting	Semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro, Zepbound), liraglutide, teriparatide, tesamorelin (Egrifta), bremelanotide (Vyleesi), insulin analogs, octreotide, leuprolide
Compounded peptides	Prescription medications prepared for an individual patient by a state-licensed pharmacy (503A) or an FDA-registered outsourcing facility (503B), using a bulk substance that the law allows	Compounding pharmacies	State board inspection; 503B facilities inspected by FDA; no FDA review of the specific product; not FDA-approved	Sermorelin, compounded semaglutide and tirzepatide (within the limits that apply after the shortages ended), NAD+ (not a peptide, but sold alongside), glutathione
"Research use only" vials	Peptides sold online with a label saying they are not for human use, while the website explains how to dose and inject them	Unregulated sellers, often reselling bulk synthesis from overseas	None that a buyer can verify; FDA treats them as unapproved drugs when marketed for human use	BPC-157, TB-500, CJC-1295, ipamorelin, GHK-Cu, "reta", "sema" and "tirz" vials
Cosmetic and food peptides	Skincare ingredients applied to the skin, or collagen fragments taken as a supplement	Cosmetics and supplement companies	Cosmetic and food regulation; no drug claims permitted	Copper peptides (GHK-Cu) in creams, Matrixyl, collagen peptides

The same compound can appear in more than one row. Semaglutide is an approved drug, a compounded drug under specific conditions, and a "research" vial. The molecule is the same; what differs is whether anyone accountable has verified what is in the vial and at what strength.

What "compounded" does and does not mean

Compounding is legal and old. Under section 503A of the Federal Food, Drug, and Cosmetic Act, a licensed pharmacist may prepare a drug for an individual patient on a prescription, using bulk substances that meet one of three tests: the substance has a recognized monograph in the United States Pharmacopeia, it is a component of an FDA-approved drug, or it appears on FDA's list of bulk substances permitted for compounding. FDA's own summary is blunt: compounded drugs are not FDA-approved, meaning FDA has not verified their safety, effectiveness or quality, and they exist for patients whose needs cannot be met by an approved product.

Sermorelin can be compounded because it was the active ingredient of an FDA-approved drug (Geref, withdrawn in 2008 for commercial reasons). Semaglutide and tirzepatide could be compounded in volume while FDA listed them as in shortage; since the shortages were declared resolved, in December 2024 for tirzepatide and February 2025 for semaglutide, pharmacies may compound them only in the narrow circumstances the law always allowed, and in March 2026 FDA sent warning letters to 30 telehealth companies over misleading marketing of compounded GLP-1s.

The opposite category is FDA's "Category 2" list: bulk substances that FDA has evaluated and found to raise significant safety risks for compounding, which pharmacies therefore do not compound. In 2023 FDA placed a group of popular peptides on that list, including BPC-157, CJC-1295 and ipamorelin. In 2026 the picture began to move. On April 15, 2026, FDA announced that twelve peptides were coming off Category 2 because their nominations had been withdrawn: BPC-157, KPV, TB-500, MOTS-c, emideltide, epitalon, semax, LL-37, dihexa, injectable GHK-Cu, PEG-MGF and melanotan II. Coming off Category 2 did not make them compoundable; they are still not on the bulks list. CJC-1295 and ipamorelin were not among the twelve, but FDA's May 14, 2026 update no longer lists them in Category 2 either: they appear as nominated but withdrawn, and they cannot be compounded. On July 23 and 24, 2026, FDA's Pharmacy Compounding Advisory Committee voted to recommend adding BPC-157, KPV and TB-500 (each 8 to 6), MOTS-c (7 to 5), epitalon (7 to 4) and semax (8 to 5) to the bulks list, and voted against emideltide (6 to 7), in each case over the objection of FDA's own review team, which had recommended against all seven. The votes are advisory; FDA must complete notice-and-comment rulemaking before anything changes, and the lawyers who follow the committee expect that to take months at least. Our legal tracker follows each compound.

What "research use only" means in practice

It means the seller is trying to avoid drug law, and in 2026 it stopped working. On August 24, 2026, FDA issued warning letters to five online peptide vendors selling semaglutide, tirzepatide, retatrutide, tesamorelin, PT-141, SS-31 and other peptides with "research use only" labels. FDA's position, stated in each letter, was that the websites showed the products were intended as drugs for human use: dosing guidance, injection instructions, reconstitution calculators and bacteriostatic water bundled with the vials. The label did not matter; the overall impression did.

For a buyer the practical problems are simpler than the legal ones. There is no pharmacy label, no verified content or purity, no sterility assurance, no beyond-use date and no one to call when something goes wrong.

Which peptides are prescribed in 2026, and what the evidence says

The list below covers the peptides a US patient is most likely to be offered through a clinic or telehealth service, with the strength of human evidence for the use it is marketed for. Grades follow our editorial policy: A is strong randomized evidence in a relevant population, D is animal or no data.

Peptides commonly prescribed or marketed in the US, 2026
Claim	Grade	What the evidence shows	Sources
Semaglutide 2.4 mg weekly for weight loss	A	STEP 1: 14.9% mean weight loss versus 2.4% with placebo at 68 weeks in 1,961 adults with obesity. FDA-approved (Wegovy).	Wilding 2021
Tirzepatide for weight loss	A	SURMOUNT-1: 15.0%, 19.5% and 20.9% mean weight loss at 5, 10 and 15 mg versus 3.1% with placebo at 72 weeks in 2,539 adults. FDA-approved (Zepbound).	Jastreboff 2022
Tesamorelin for abdominal fat	B	Approved for HIV-associated abdominal fat accumulation: about 15% reduction in visceral fat over 26 weeks. Use in people without HIV is extrapolation.	Falutz 2007
Sermorelin for GH and IGF-1 in older adults	B	Small trials (10 to 19 participants, 2 to 16 weeks) show higher GH output and, in some, higher IGF-1; body-composition changes modest. No long-term outcome data.	See the sermorelin profile
Bremelanotide (PT-141) for low sexual desire	B	FDA-approved (Vyleesi) for premenopausal women with hypoactive sexual desire disorder on the basis of randomized trials; evidence in men is limited to small studies.	FDA label (Vyleesi)
Glutathione injections for detox or skin lightening	C	A natural tripeptide in FDA's Category 1, legally compoundable while FDA evaluates it; human evidence for the marketed uses is small and inconsistent.	FDA categories 2026
BPC-157 for healing and injuries	D	Hundreds of rodent studies from essentially one research group; no adequate human trials. FDA reviewers in July 2026 recommended against listing it; the advisory committee voted 8 to 6 to recommend it anyway.	HPM 2026a
TB-500 (thymosin beta-4 fragment) for recovery	D	At the July 2026 meeting FDA acknowledged there are no studies on TB-500 itself, and several panelists raised a possible cancer signal.	HPM 2026a
CJC-1295 and ipamorelin for anti-aging	D	Pharmacology studies show growth hormone release in small numbers of adults; no trials of clinical outcomes. FDA flagged both as compounding safety risks in 2023, and its advisory committee voted 12 to 0 against ipamorelin, with one abstention (October 2024) and 13 to 0 against CJC-1295 (12 to 1 for its acetate form; December 2024).	FDA PCAC 2024

A = strong human trials or meta-analyses · B = small or short human trials · C = observational, mechanistic or indirect human data · D = animal/in-vitro only, or no evidence. Grades describe the evidence, not whether we recommend the use.

Two things stand out. First, the gap between the top and the bottom of the table is not a matter of degree; the GLP-1 drugs have some of the largest and best-run trials in medicine, while the peptides with the loudest marketing have none. Second, the compounds in the bottom half are also the ones you cannot get from a licensed pharmacy, which means the people using them are relying on unlabeled vials.

What peptides do, in plain language, by goal

Weight and metabolic health: GLP-1 and GLP-1/GIP drugs reduce appetite, slow gastric emptying and improve insulin response. They are the reason "peptide" became a household word.
Growth hormone and body composition: GHRH analogs (sermorelin, tesamorelin) and ghrelin-receptor agonists (ipamorelin, and the oral drug MK-677) raise growth hormone output. Growth hormone itself is a 191-amino-acid protein and is not a "peptide" in the marketing sense; its distribution for non-approved uses is a federal crime under 21 U.S.C. 333(e), which is why clinics prescribe secretagogues instead.
Sexual function: Bremelanotide acts on melanocortin receptors in the brain, not on blood vessels like sildenafil.
Healing and inflammation: BPC-157, TB-500, KPV and similar compounds are the focus of the 2026 regulatory debate. The claims come from animal data.
Skin: Copper peptides and other signal peptides in cosmetics have modest evidence for wrinkle appearance; injectable GHK-Cu has none.
"Longevity": Epitalon, MOTS-c, SS-31 and others are marketed on mechanisms (telomerase, mitochondria) with little or no human outcome data.

Safety: what actually goes wrong

The documented harms in practice cluster in three groups.

Dosing errors with compounded GLP-1s. Compounded semaglutide and tirzepatide are dispensed in vials and drawn up in units, and FDA has received reports of patients taking many times the intended dose because of unit-to-mg confusion. Our units guide exists for this reason.
Unregulated products. With no verified content, a "research" vial can be underdosed, overdosed, contaminated, or a different compound altogether. Immunogenicity (the immune system reacting to impurities or aggregated peptide) and poor characterization were FDA's stated concerns for the peptides it reviewed in 2026; for TB-500, FDA said the exact sequence and molecular weight vary by vendor.
The predictable effects of the hormone system being stimulated. Nausea, vomiting and gallbladder problems with GLP-1s; fluid retention, joint pain, carpal tunnel symptoms and reduced insulin sensitivity with growth hormone secretagogues; flushing and nausea with bremelanotide. These are on the labels and in the trials, and a prescribing clinician should screen for them.

Peptides do not carry the liver, cardiovascular and endocrine toxicity of anabolic steroids, and they are not addictive. They are also not inert, and "natural" does not describe a molecule engineered to last a week in your bloodstream.

How to get a peptide prescribed, and what to ask

The legal, accountable route is the same for every prescription drug: a licensed clinician evaluates you, decides a compound is appropriate, writes a prescription, and a licensed pharmacy or a manufacturer dispenses a labeled product. Telehealth has made this easy for the compounds that can be legally prescribed. Questions worth asking any clinic:

Which pharmacy will fill this, in which state, and is it a 503A pharmacy or a 503B outsourcing facility?
Is this compound currently permitted for compounding? (If the clinic says a peptide that is not on the bulks list is "fully legal", the product is probably a research vial.)
What is the evidence for this compound for my goal, and what will we measure to know whether it is working?
What are the dose, the concentration and the units, written on the label, and who do I call about side effects?

Bottom line

Peptides are the body's own messaging molecules, and peptide drugs are those messages engineered to last. A few of them are among the best-proven medicines of the past decade; many of the ones sold online have never been tested in a human trial. The word "peptide" tells you nothing about which kind you are being offered. The source, the label and the evidence do.

Frequently asked questions

+Are peptides steroids?

No. Anabolic steroids are synthetic versions of testosterone, a small fat-soluble molecule that enters cells and binds a receptor in the nucleus. Peptides are chains of amino acids that act on receptors on the cell surface, are broken down by enzymes within minutes to hours, and are not controlled substances in the United States. Some peptides raise growth hormone, which is a separate hormone with its own rules; growth hormone itself is tightly restricted by federal law.

+Are peptides safe?

There is no single answer, because the word covers FDA-approved drugs with decades of safety data (insulin, semaglutide, teriparatide), compounded drugs prepared from approved ingredients with limited data for the way they are used, and unregulated vials with no data at all. The safety question is always about a specific compound, dose, source and person. The best-documented harms in practice are dosing errors with compounded GLP-1s, contamination and mislabeling of unregulated products, and the known side effects of the hormone systems a peptide stimulates.

+Are peptides legal in the United States?

Peptides are not controlled substances. Whether a given peptide can be legally prescribed depends on whether it is an approved drug, whether a compounding pharmacy may legally prepare it, and whether FDA has flagged it as a safety risk for compounding. Selling vials for human use without approval is illegal regardless of a research-use-only label, as the FDA warning letters of August 2026 made explicit. Our legal tracker covers each major peptide.

+Do peptides work for weight loss?

The GLP-1 class does, with the strongest evidence of any weight-loss medication: an average loss of about 15% of body weight with semaglutide 2.4 mg and up to about 21% with tirzepatide 15 mg over 68 to 72 weeks in randomized trials. No other peptide marketed for weight loss has comparable human evidence; several have none.

+What is the difference between a peptide and a protein?

Length, by convention. Chains of roughly 2 to 50 amino acids are called peptides; longer chains that fold into complex structures are proteins. Insulin, at 51 amino acids in two chains, sits at the boundary and is often called a peptide hormone. The distinction matters less than the function: both are signals or workers made of the same building blocks.

+Are collagen peptides the same thing as peptide therapy?

No. Collagen peptides are a food supplement: collagen broken into fragments that are digested like any other protein. Topical cosmetic peptides are skincare ingredients. Peptide therapy refers to prescription medications, usually injected, that act as hormones or signals in the body.

+Are peptides natural?

Most therapeutic peptides are copies or modified versions of molecules the body already makes: GLP-1, growth hormone-releasing hormone, oxytocin, insulin. Modification is usually what makes them useful as drugs, because the natural versions are destroyed in minutes. Semaglutide, for example, is human GLP-1 with two amino-acid changes and a fatty-acid chain that keeps it in circulation for a week.

+How are peptides taken?

Most are injected under the skin, because the digestive system breaks peptides down before they can be absorbed. Exceptions exist: an oral semaglutide tablet uses an absorption enhancer, some peptides are given as nasal sprays, and topical peptides are used in skincare. Oral peptide capsules sold as supplements are digested like food.