How Do Peptides Work? Receptors, Signaling, and Why Delivery Matters

How do peptides work?

Peptides are short chains of amino acids that act as chemical messengers. A therapeutic peptide binds a specific receptor on the cell surface, commonly a G-protein coupled receptor, and that binding sets off a signaling cascade inside the cell that changes what the cell does. Because digestion destroys most peptides, they are usually injected, a few are given nasally, and many are chemically modified to last longer.

What a peptide is (and what is not one)

A peptide is a chain of amino acids weighing roughly 500 to 5,000 daltons, between small-molecule drugs and large proteins. Insulin, synthesized in 1921, was the first therapeutic peptide. Per a 2022 review in Signal Transduction and Targeted Therapy, more than 80 peptide drugs have since been approved worldwide and more than 170 are in active clinical development.

Several compounds sold alongside peptides are not peptides. NAD+ is a coenzyme, not a receptor-binding messenger. Testosterone is a steroid hormone that acts on a receptor inside the cell rather than on its surface. Enclomiphene is a small-molecule selective estrogen receptor modulator, and no product whose active ingredient is enclomiphene alone is FDA-approved, though enclomiphene is one of the two isomers in FDA-approved clomiphene citrate. Glutathione is technically a tripeptide, but it acts as an antioxidant rather than a receptor-signaling drug. Where clinics dispense prescription versions of these compounds, they are compounded preparations, which per FDA are not FDA-approved: FDA does not verify their safety, effectiveness, or quality before marketing.

Step one: the peptide finds its receptor

In a 2018 analysis cited by the 2022 review, more than 90 percent of peptides then in clinical development acted on targets outside the cell, including G-protein coupled receptors (GPCRs), the gonadotropin-releasing hormone receptor, and the GLP-1 receptor. The GLP-1, GHRH, and melanocortin receptors below are all GPCRs. The receptor is the lock, the peptide the key: semaglutide, per the Ozempic label, selectively binds and activates the GLP-1 receptor, sharing 94 percent of its sequence with the hormone it copies. Binding switches on a G protein inside the membrane, which switches on an enzyme that makes a second messenger, usually cyclic AMP (cAMP).

Not every peptide uses a GPCR, and insulin is the standard exception. Its receptor also sits on the cell surface, which keeps insulin inside that extracellular-target majority, but it is a receptor tyrosine kinase. Per an NIH StatPearls review, the insulin receptor’s beta subunits carry tyrosine kinase activity: binding switches on that activity in the receptor itself rather than handing the signal to a G protein, and the cascade that follows ends with muscle and fat cells taking up more glucose.

Step two: the signal inside the cell, three worked examples

GLP-1 receptor agonists: semaglutide and tirzepatide

GLP-1 is a gut hormone released after meals, and its receptor is a 463-amino-acid GPCR. In pancreatic beta cells, activation raises cAMP, which turns on protein kinase A and Epac2, which close ATP-sensitive potassium channels so insulin is released only when glucose is high. GLP-1 also suppresses glucagon, slows stomach emptying, and acts on hypothalamic satiety circuits, per a 2017 review in Annals of Pediatric Endocrinology and Metabolism. Tirzepatide adds a second target: the Zepbound label calls it an agonist at both the GIP and GLP-1 receptors, which are both present in brain areas involved in appetite. Compounded semaglutide and compounded tirzepatide are not FDA-approved, and FDA has not evaluated them for safety, quality, or efficacy.

Sermorelin: the GHRH receptor and pulsed growth hormone

Per a 2025 review in Reviews in Endocrine and Metabolic Disorders, the full biological activity of GHRH is contained in its first 29 amino acids, GHRH(1-29). That sequence is dispensed as sermorelin: Drugs@FDA lists sermorelin acetate as the active ingredient of Geref. The GHRH receptor is a seven-transmembrane GPCR on pituitary somatotroph cells. Activation drives growth hormone release through cAMP-dependent pathways and restocks the cell for the next pulse. The review also notes that uninterrupted or recurring stimulation of the receptor leads to attenuation of growth hormone release, through receptor desensitization.

No FDA-approved sermorelin product is marketed in the United States today: per Drugs@FDA, Geref was approved in 1997 and its products are now listed as discontinued. Every sermorelin prescription is therefore filled as a compounded preparation, which means it is not FDA-approved and FDA has not evaluated it for safety, quality, or efficacy.

PT-141 (bremelanotide): melanocortin receptors in the brain

Bremelanotide is a cyclic heptapeptide and melanocortin receptor agonist. The Vyleesi label states that binding to MC1R and MC4R is most relevant at therapeutic doses, that MC4R-expressing neurons sit throughout the central nervous system, and that the mechanism by which the drug improves hypoactive sexual desire disorder (HSDD) is unknown. One peptide can fit several locks: MC1R on melanocytes explains the drug’s pigmentation effects. Vyleesi is FDA-approved only for premenopausal women with acquired, generalized HSDD; compounded PT-141, including nasal forms and any use in men, is off-label and not FDA-approved.

Half-lives below come from FDA labels, except native GLP-1, which comes from the 2017 review; sermorelin has no current FDA label because no FDA-approved sermorelin product is marketed (it is compounded only, and FDA has not evaluated compounded drugs for safety, quality, or efficacy); insulin varies by product.

Peptide	Receptor (type)	Main downstream effect	Способ применения	Half-life
Native GLP-1	GLP-1 receptor (GPCR)	cAMP rise: insulin only when glucose is high, less glucagon, slower emptying, satiety	Released by the gut	About 2 minutes
Semaglutide	GLP-1 receptor (GPCR)	Same pathway as native GLP-1	Subcutaneous weekly; oral daily (Rybelsus, oral Wegovy)	About 1 week (injection)
Tirzepatide	GIP and GLP-1 receptors (two GPCRs)	GLP-1 pathway plus GIP signaling in appetite areas	Subcutaneous weekly	About 5 to 6 days
Sermorelin (GHRH 1-29)	GHRH receptor (GPCR)	cAMP-dependent growth hormone release	Subcutaneous	No current FDA label (compounded only)
Bremelanotide (PT-141)	Melanocortin MC1R, MC4R (GPCR)	CNS signaling; HSDD mechanism unknown	Subcutaneous, on demand	About 2.7 hours
Insulin	Insulin receptor (receptor tyrosine kinase)	Receptor kinase activation inside the cell; more glucose uptake by muscle and fat	Subcutaneous	Varies by product

Why swallowing most peptides does not work

Peptides have two built-in weaknesses, per the 2022 review: they cannot cross cell membranes on their own, and enzymes easily break their amide bonds. A 2026 review in Frontiers in Drug Delivery lists the gauntlet: pepsin, then trypsin, chymotrypsin, and elastase, then brush-border peptidases, then the gut wall and liver. Oral bioavailability for peptides meant to act body-wide is typically under 1 percent.

Oral semaglutide is co-formulated with an absorption enhancer, salcaprozate sodium (SNAC), which protects the peptide from pepsin and helps it cross the stomach lining. Per the Frontiers review, SNAC is the enhancer in every approved oral semaglutide product, including the oral Wegovy 25 mg approved in December 2025, and absolute bioavailability is still only about 0.4 to 1 percent, against 89 percent for injected semaglutide per the Ozempic label, and only on an empty stomach. Octreotide and desmopressin also have approved oral or sublingual forms. Peptide capsules sold online for compounds such as BPC-157 or CJC-1295 have no published human absorption data.

Why injections and nasal sprays exist

Subcutaneous injection skips digestion. Per their FDA labels, absolute bioavailability is 89 percent for semaglutide, 80 percent for tirzepatide, and about 100 percent for bremelanotide, peaking at 1 to 3 days, a median 24 hours, and about an hour.

Nasal delivery skips both the needle and the gut for a handful of peptides. The Frontiers review notes that nasal desmopressin is long established, nasal calcitonin was approved for osteoporosis, and nasal glucagon reaches roughly 30 to 40 percent of its dose; it frames the nasal route as a fit for very potent peptides that need only small doses, not a general substitute for injection. No compounded nasal peptide, including nasal PT-141, has FDA-reviewed bioavailability data or FDA approval. See PT-141 nasal spray versus injection для получения дополнительной информации.

Half-life engineering: from two minutes to one week

The body’s own GLP-1 survives about 2 minutes: dipeptidyl peptidase-4 (DPP-4) snips off its first two amino acids, and neutral endopeptidase finishes the job. Long-acting agonists get around that. Exenatide, the first GLP-1 receptor agonist approved, copies exendin-4, a gila monster saliva hormone that shares 53 percent of its sequence with GLP-1. Per Lee and Lee (2017), it happens to carry glycine where DPP-4 cleaves GLP-1 at position 8, so the enzyme cannot cut it, and its half-life is about 2 to 4 hours.

Semaglutide, by contrast, stacks deliberate modifications listed in the Ozempic label: a position-8 change that blocks DPP-4, plus a hydrophilic spacer and a C18 fatty di-acid on lysine 26 that bind albumin. Bound to albumin (more than 99 percent of the drug), it is too large to filter and shielded from enzymes. That gives a half-life of about 1 week and steady state after 4 to 5 weeks. Tirzepatide uses the same playbook: per the Zepbound label, aminoisobutyric acid at positions 2 and 13 and a C20 fatty diacid on lysine 20 (99 percent albumin bound) give it about 5 to 6 days.

Bremelanotide shows the opposite design goal: its ring structure and a D-phenylalanine add stability, yet its half-life is only about 2.7 hours, which suits an on-demand drug rather than a standing schedule.

Which peptides are FDA-approved, compounded, or not permitted

Mechanism says nothing about regulatory status. In the U.S., peptides fall into three groups.

FDA-approved products

Semaglutide (Ozempic, Wegovy, Rybelsus), tirzepatide (Mounjaro, Zepbound), bremelanotide (Vyleesi), and insulin products carry FDA-reviewed labels. Sermorelin and enclomiphene are absent from this list for two different reasons: Geref, the approved sermorelin product, is discontinued, and no product whose active ingredient is enclomiphene alone has ever been FDA-approved, though enclomiphene is one of the two isomers in FDA-approved clomiphene citrate, which matters because being a component of an FDA-approved drug product is one of the section 503A conditions below.

Compounded prescription peptides

A 503A pharmacy prepares a medication for an individual patient with a prescription. Per FDA, compounded drugs are not FDA-approved: FDA does not verify their safety, effectiveness, or quality before they are marketed. Sermorelin now reaches patients only in this form. FDA declared the tirzepatide shortage resolved on December 19, 2024, and the semaglutide shortage resolved on February 21, 2025. Enforcement discretion for compounded copies ended between February and May 2025, so compounded GLP-1s are now permitted only when a prescriber documents a change that makes a significant difference for an identified patient, never as a routine alternative.

Peptides not eligible for compounding

Here the rule matters more than the list. Per FDA, under section 503A of the Federal Food, Drug, and Cosmetic Act, a pharmacy may compound only with a bulk drug substance that complies with an applicable United States Pharmacopeia (USP) or National Formulary (NF) monograph if one exists; otherwise is a component of an FDA-approved drug product; otherwise appears on FDA’s list of bulk drug substances that can be used in compounding, the 503A bulks list. The research peptides sold online meet none of the three conditions.

The safety history is a separate matter. FDA’s interim policy sorts the substances nominated for the 503A bulks list into three categories: Category 1, no safety risks identified, where FDA does not intend to act against a pharmacy that compounds with the substance while the evaluation continues; Category 2, where FDA identified significant safety risks; and Category 3, nominated with too little information to evaluate. The prescription peptides Revive does offer are compounded under that framework; BPC-157, TB-500, CJC-1295, MOTS-c, AOD-9604, and ipamorelin are not eligible at all. FDA previously placed BPC-157, thymosin beta-4 (TB-500), CJC-1295, MOTS-c, and AOD-9604 in Category 2, citing immunogenicity risk, peptide impurities, and missing human safety data. On FDA’s safety-risk page, current as of April 2026, those five sit in a separate table of nominations withdrawn by the nominators. Peptide entries still in the active Category 2 table include kisspeptin-10 (503A) and ipamorelin acetate, GHRP-2, and GHRP-6 (503B), all added September 29, 2023.

Withdrawing a nomination is not permission: it removes a substance from consideration for the 503A bulks list, so it still fails all three conditions above. Revive does not offer any of these peptides.

Before buying, verify the provider’s and the pharmacy’s state licenses, look for LegitScript certification, and expect labeling that says a compounded product is not FDA-approved. See the FDA peptide review и how to choose a licensed peptide clinic.

How Revive approaches peptide therapy

Revive Longevity is a LegitScript-certified telehealth clinic with providers licensed in all 50 states. Every treatment begins with an evaluation by a licensed provider, and the peptide therapies described here are prescription-only: the provider decides whether a peptide is appropriate, which one, and by what route. Content is medically reviewed by Diana Medina, PA-C, a board-certified Physician Associate.

Revive’s peptide options are limited to prescription treatments such as compounded sermorelin ($179) and compounded PT-141 ($199), and, only where a prescriber documents an individual clinical need, compounded tirzepatide ($279). These are compounded preparations: they are not FDA-approved, and FDA has not evaluated them for safety, quality, or efficacy. Prices are per 4 weeks and subject to change and to eligibility. Any compounded prescription is prepared by a licensed 503A pharmacy. Revive does not offer BPC-157, TB-500, CJC-1295, ipamorelin, MOTS-c, AOD-9604, or other peptides ineligible for compounding under section 503A. Nothing here promises results. See how Revive works for the process.

Часто задаваемые вопросы

Do peptides work if you take them as a pill?

Mostly no. Digestive enzymes break peptides apart and the gut wall blocks most of what survives, so oral bioavailability is typically under 1 percent. Oral semaglutide is the best-known exception, and even with an absorption enhancer and strict empty-stomach rules it reaches only about 0.4 to 1 percent.

How can semaglutide last a week when natural GLP-1 lasts two minutes?

Natural GLP-1 is cut apart within minutes by DPP-4. Semaglutide has a modified position 8 that blocks DPP-4 and a C18 fatty di-acid on lysine 26 that binds albumin, so more than 99 percent of the drug travels shielded from enzymes and kidney filtration. Its half-life is about 1 week per the Ozempic label.

Are peptide nasal sprays as effective as injections?

It depends on the molecule. Nasal glucagon reaches roughly 30 to 40 percent of its dose, while the subcutaneous peptides here reach 80 to 100 percent; those are different molecules, so treat the contrast as illustrative. A 2026 review frames the nasal route as a fit for very potent peptides that need small doses, not a drop-in substitute. No compounded nasal peptide, including nasal PT-141, is FDA-approved.

Are NAD+, testosterone, and enclomiphene peptides?

No. NAD+ is a coenzyme, testosterone is a steroid hormone that acts on a receptor inside the cell, and enclomiphene is a small-molecule selective estrogen receptor modulator. None uses the surface-receptor cascade described here. No product whose active ingredient is enclomiphene alone is FDA-approved, though enclomiphene is one of the two isomers in FDA-approved clomiphene citrate. The prescription versions clinics dispense are compounded preparations, which per FDA are not FDA-approved.

Can I get BPC-157 or TB-500 from a licensed clinic?

No. Per FDA, a pharmacy may compound with a bulk substance only if it complies with a USP or NF monograph where one exists, otherwise is a component of an FDA-approved drug product, otherwise appears on FDA’s 503A bulks list. BPC-157 and TB-500 meet none of those, so neither may be compounded. FDA previously placed both in Category 2 for significant safety risks, and those nominations were later withdrawn, which removes a substance from consideration rather than granting permission. Revive does not offer them.