What is a nonapeptide? Definition and examples
Clinical review pending
Nonapeptides contain nine amino acids. Explore oxytocin, vasopressin, bradykinin, and cosmetic Nonapeptide-1, including their structures, medical uses, and limits of the evidence.
Nonapeptides are short chains of nine amino acids. Their length tells you little about what they do: some help regulate water balance or childbirth, while others participate in inflammation. The synthetic skin-care ingredient Nonapeptide-1 belongs to the same size category, but it has a different purpose and much more limited clinical evidence behind its use.
Key takeaways
- A nonapeptide contains nine amino acid residues joined by eight peptide bonds in a linear backbone. It is an oligopeptide, a term commonly used for short chains of roughly 2–20 residues, although the boundary is a convention. The cyclic hormones discussed here also have a disulfide bridge, which is not a peptide bond.
- Oxytocin and arginine vasopressin are the two classic human nonapeptide hormones. Both are cyclic. They differ at just two amino acid positions but have distinctly different physiological roles.
- Bradykinin is a linear nonapeptide that widens blood vessels and makes their walls more permeable.
- “Nonapeptide” names a size category. “Nonapeptide-1” names a particular synthetic cosmetic ingredient. Cosmetic claims concern appearance; disease-treatment claims fall into a different regulatory category.
- Some nonapeptides are approved medicines. In the United States, oxytocin is FDA-approved for uses including medically indicated labor induction and control of bleeding after delivery; vasopressin is approved for vasodilatory shock. Both require a prescription.
- Blood measurements of oxytocin and vasopressin are largely research or specialist tests, rather than part of a routine blood panel.
What is a nonapeptide?
A nonapeptide is a peptide made of nine amino acid residues. Its name combines the Latin nona, meaning “ninth,” with the Greek peptós, meaning “digested.” In a linear chain, eight peptide bonds connect the nine building blocks. In a simplified chemical accounting of bond formation, each condensation releases one water molecule. That is not a literal description of every step in cellular protein synthesis.
Nonapeptides belong to the oligopeptide category, commonly described as chains of 2–20 amino acids. The body’s cyclic nonapeptide hormones have molecular masses of about 1,000 daltons. A dalton is a unit of atomic and molecular mass: oxytocin is about 1,007 daltons, and arginine vasopressin about 1,084. Forming a disulfide bridge removes two hydrogen atoms, slightly decreasing rather than increasing mass.
The two best-known human nonapeptide hormones, oxytocin and arginine vasopressin, are cyclic. A disulfide bond connects the cysteine residues at positions 1 and 6, often written Cys1 and Cys6. This stabilizes a shape important for receptor recognition. The bond joins two cysteine side chains: it neither adds a ninth peptide bond nor connects the two ends of the amino acid backbone. Oxytocin belongs to the neurohypophyseal peptide hormone family and has functions both in the central nervous system and elsewhere in the body. “Oxytocin in health and disease” reviews these roles.
Nonapeptide versus Nonapeptide-1: similar names, different meanings
“Nonapeptide” can describe any nine-amino-acid peptide, including the body’s own oxytocin and arginine vasopressin. Nonapeptide-1, by contrast, is the name of one synthetic molecule used as a cosmetic ingredient.
The US FDA framework for cosmetics distinguishes appearance-related claims from claims to treat disease. These similar names therefore cover different molecules, routes of use, and bodies of evidence.
How a nonapeptide’s structure forms
In a simplified chemical diagram, eight condensation reactions join nine amino acids. They form eight amide bonds—the peptide bonds—with the loss of eight water molecules. In the biologically important cyclic hormones discussed here, the cysteine residues at positions 1 and 6 are also joined through oxidation to form a disulfide bond.
This posttranslational modification creates a ring within the chain and restricts its three-dimensional arrangement. Oxytocin is the classic example: its two cysteines form a disulfide bond within the same molecule. Its terminal glycine is also amidated, carrying Gly-NH₂ rather than a free carboxylate group. Both oxytocin and vasopressin have this additional posttranslational modification.
How nonapeptides compare with neighboring peptide sizes
Moving from an octapeptide with eight amino acids to a nonapeptide adds one residue and one peptide bond to a linear backbone. For oxytocin and vasopressin, however, the ring structure is more consequential than that extra building block. It depends on the placement of cysteines that can form a disulfide bridge; amino acid count alone does not determine it.
Decapeptides contain ten amino acids. They include natural signaling peptides and linear synthetic cosmetic peptides such as Decapeptide-12. In this particular comparison, architecture is a useful distinction: the classic nonapeptide hormones are cyclic, whereas Decapeptide-12 is linear. It is not a general rule that one chain length must produce one structure.
How nonapeptides compare with related peptides
| Peptide class | Amino acids | Peptide bonds | Size category | Example | Main biological role or proposed use |
|---|---|---|---|---|---|
| Dipeptide | 2 | 1 in a linear chain | Oligopeptide | Carnosine (β-Ala-His) | pH buffering and antioxidant functions in muscle and brain |
| Tetrapeptide | 4 | 3 in a linear chain | Oligopeptide | Tuftsin (Thr-Lys-Pro-Arg) | Stimulation of innate immune functions, particularly phagocytosis |
| Pentapeptide | 5 | 4 in a linear chain | Oligopeptide | Met-enkephalin (Tyr-Gly-Gly-Phe-Met) | Opioid receptor binding and modulation of pain signals |
| Nonapeptide | 9 | 8 in the backbone; the cyclic examples here also have a disulfide bridge | Oligopeptide | Oxytocin, arginine vasopressin | Hormonal signaling involved in social behavior, childbirth, kidney water regulation, and vascular tone |
| Decapeptide | 10 | 9 in a linear chain | Oligopeptide | Decapeptide-12 | Tyrosinase inhibition and reduced melanin synthesis as a cosmetic brightening approach |
| Oligopeptide generally | Commonly 2–20 | 1–19 in a linear chain | Oligopeptide | The examples above and fragments of insulin C-peptide | Molecule-dependent: signaling, antimicrobial activity, and cosmetic or nutritional applications |
The bond counts refer to the linear backbone. The additional disulfide bridge in oxytocin and vasopressin changes their shape without changing the number of peptide bonds.
Examples of nonapeptides in human biology
Oxytocin and vasopressin are familiar members of this size category, and their molecular family has been strongly conserved through evolution. Nonapeptide biology extends beyond these hormones to inflammation and activity in bone cells. Comparisons across species also reveal how many different tasks these molecules perform.
Oxytocin
Oxytocin has the sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂, with a disulfide bridge between positions 1 and 6. Large neurosecretory cells called magnocellular neurons produce it in the hypothalamus’s paraventricular and supraoptic nuclei. It is released into the bloodstream through the posterior pituitary.
In the central nervous system, oxytocin influences maternal, sexual, social, and stress-related behavior. Outside the brain, it stimulates smooth muscle contraction, including uterine contractions and milk ejection from the breast. Milk ejection is distinct from milk production. A whole-mouse-brain atlas at the single-transcript level mapped oxytocin and its receptor. It provides anatomical and molecular clues, rather than proof that a particular pattern causes a human behavior.
Oxytocin is also FDA-approved for medically indicated labor induction and control of bleeding after childbirth. The cited US prescribing information specifically excludes purely elective induction without a medical indication. Oxytocin is therefore an example of a natural nonapeptide used directly as a medicine. The Superpower oxytocin guide provides further background.
Plasma oxytocin measurement is not a routine clinical test. Cortisol, prolactin, and estradiol may be measured for other clinical questions involving interacting hormone systems, but they do not substitute for an oxytocin measurement.
Arginine vasopressin (AVP)
Arginine vasopressin has the sequence Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH₂. Also called antidiuretic hormone, or ADH, it is the second classic human nonapeptide hormone.
It differs from oxytocin at just two positions: phenylalanine replaces isoleucine at position 3, and arginine replaces leucine at position 8. Yet its main functions are quite different. Through V2 receptors in the kidney’s collecting ducts, AVP promotes water reabsorption; through vascular V1a receptors, it constricts blood vessels. A review of vasopressin physiology and clinical use discusses these functions. AVP also participates in the endocrine stress response, as described in the review of copeptin and fluid regulation.
Vasopressin injection is FDA-approved to increase blood pressure in adults with vasodilatory shock whose pressure remains low despite fluids and catecholamines. In this form of shock, blood vessels are excessively dilated.
Terlipressin, sold as Terlivaz, is approved in the United States to improve kidney function in adults with hepatorenal syndrome and rapidly declining kidney function. It is a related vasopressin analog, but contains 12 amino acids, so terlipressin itself is not a nonapeptide. Both medicines require a prescription and hospital care.
In clinical practice, plasma sodium and serum and urine osmolality help assess the water balance that AVP regulates. Osmolality describes the concentration of osmotically active dissolved particles. These measurements provide context, not a direct AVP reading.
Oxytocin and vasopressin: only two residues apart
| Feature | Oxytocin | Arginine vasopressin |
|---|---|---|
| Amino acid sequence | Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂ | Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH₂ |
| Different positions | Ile at position 3, Leu at position 8 | Phe at position 3, Arg at position 8 |
| Receptors | Oxytocin receptor (OXTR) | V1a in blood vessels, V2 in the kidney |
| Main target tissues | Uterus, breast, brain | Kidney collecting ducts, vascular smooth muscle |
| Main physiological effects | Childbirth, milk ejection, and participation in social bonding | Kidney water reabsorption and vasoconstriction |
| FDA-approved drug formulation | Oxytocin injection for medically indicated labor induction | Vasopressin injection for vasodilatory shock |
Bradykinin: a nonapeptide that is not a classic hormone
Bradykinin is another important human peptide with nine residues: Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg. Unlike the classic nonapeptide hormones, it is linear and has no ring-closing disulfide bridge.
Bradykinin acts mainly as a local inflammatory mediator, rather than a classic endocrine signal traveling through the bloodstream. It increases blood vessel permeability and dilates vessels, and it contributes to pain signaling.
Nonapeptides in comparative and evolutionary biology
The oxytocin–vasopressin family is a highly conserved vertebrate signaling system. In many nonmammalian vertebrates, arginine vasotocin (AVT) and mesotocin have roles corresponding to vasopressin and oxytocin. Isotocin is an oxytocin-like peptide found in teleost fish. These are cyclic nonapeptides with the same basic architecture, although species differ in their peptide systems.
The brain distribution of oxytocin-like and vasopressin-like peptides is broadly similar across vertebrate groups, while receptor distributions vary considerably. In a cichlid species with care by both parents, brain vasotocin was associated with parental care. This adds to evidence that nonapeptides participate in social behavior across very different animals.
Arginine vasotocin and effects across tissues
Vasopressin-family peptides act beyond the kidney and their other familiar hormonal targets. A cell study found that AVP increased calcium channel currents in osteoblasts, the cells that build bone. Researchers used patch-clamp recording to measure electrical currents through the cell membrane. This demonstrates activity in bone cells, alongside AVP’s established kidney and cardiovascular functions; it does not establish a bone treatment.
Experiments with vasotocin analogs in rats also showed that small amino acid substitutions can separate water-retaining effects from effects on sodium and potassium excretion. Different vasopressin receptor subtypes were implicated. Neither cell recordings nor rat experiments establish clinical efficacy for a corresponding use in humans.
Examples of nonapeptides in medicine and cosmetics
Nine-amino-acid sequences appear in very different settings. Natural hormones are used as prescription medicines, while cosmetic approaches aim to make skin tone look brighter or more even.
Nonapeptide-1 for cosmetic skin brightening
Nonapeptide-1 is a synthetic nine-amino-acid sequence developed to inhibit the synthesis of melanin, a major skin pigment. Its cosmetic ingredient name is Nonapeptide-1; it is also known as Melanostatine-5. It has CAS number 158563-45-2 and the sequence Met-Pro-D-Phe-Arg-D-Trp-Phe-Lys-Pro-Val-NH₂. The PubChem entry for Nonapeptide-1 describes the molecule.
Its proposed mechanism involves competing with alpha-melanocyte-stimulating hormone, or alpha-MSH, at the melanocortin-1 receptor (MC1R). Other cosmetic brightening approaches, including Decapeptide-12, focus on directly inhibiting the enzyme tyrosinase. Reviews of tyrosinase inhibitors from a medicinal chemistry perspective and peptide and peptidomimetic tyrosinase inhibitors explain that broader field.
Agouti signal protein provides a biological model for receptor competition. It naturally competes with alpha-MSH at MC1R and shifts pigment synthesis from eumelanin toward pheomelanin. A study of its bioactive motifs examined agouti-derived peptides in mouse pigment cells. It was not a clinical trial of Nonapeptide-1. Mimicking this receptor biology is the cosmetic hypothesis; whether a finished product visibly changes skin tone needs its own evidence.
In an open-label study of a Decapeptide-12 skin-care system, ratings of photodamage improved among the 13 participants who completed the 24-week study, out of 15 enrolled. The system also included a glycolic-acid moisturizer, broad-spectrum sunscreen, and an antioxidant cleanser. There was no control group. The combination and the different peptide mean this is not direct evidence of efficacy for Nonapeptide-1.
Direct clinical evidence in humans specifically for Nonapeptide-1 remains limited. Under the FDA cosmetics framework, cosmetic claims concern appearance. Claims to treat disease or affect body structure or function can make a product a drug under US law.
Oxytocin as a nonapeptide medicine
Oxytocin is an FDA-approved nonapeptide medicine. It is administered by intravenous infusion for medically indicated induction or augmentation of labor. An infusion or intramuscular injection may be used to control uterine bleeding after delivery. These specific uses have an extensive clinical evidence and experience base from decades of obstetric practice.
Research involving autistic people addresses a separate question. A 2010 review of translational treatment approaches describes early attempts to turn biological findings into therapies. Later trials give a more informative, mixed picture. In a 2021 randomized trial involving 290 children and adolescents aged 3–17, 24 weeks of intranasal oxytocin did not improve the primary social-withdrawal outcome compared with placebo; secondary outcomes were also not meaningfully different. A smaller crossover pilot involving 41 children aged 3–8 reported improvements on autism-related measures with six weeks of every-other-day oxytocin paired with positive social interaction. That encouraging, protocol-specific result does not establish general effectiveness or override the larger negative trial.
Approval of injectable oxytocin for obstetric care does not establish an approved psychiatric use for a compounded nasal product. Compounded drugs are not FDA-approved; the agency does not review their safety, effectiveness, or quality before marketing. Psychiatric applications discussed here remain investigational rather than a basis for self-treatment.
Nasal delivery is scientifically interesting. In a rat study, most oxytocin reaching the brain after nasal administration arrived directly from the nasal cavity rather than through the bloodstream. This describes transport in an animal model, not an effective psychiatric treatment in humans.
Peptide combinations in cosmetic formulations
Cosmetic products often combine several peptides. One eye cream combined palmitoyl tripeptide-1 and palmitoyl tetrapeptide-7 with two nonpeptide ingredients. The eye-cream study assessed skin-quality measures rather than pigmentation outcomes. A separate paper reviews signal peptides as cosmetic ingredients.
These signaling peptides are being investigated as a promising cosmetic ingredient category. Peptides and other marine-derived bioactive substances are also being explored for cosmetic purposes, including hyperpigmentation. The Superpower tripeptide guide offers further background. Studies of tri- and tetrapeptide formulations, however, are not direct tests of Nonapeptide-1.
Can nonapeptide levels be measured?
Yes, particular nonapeptides can be measured with specialized methods. These are not standard components of routine blood testing.
Plasma oxytocin is mainly measured in research, and the method matters greatly. Immunoassays performed with and without prior sample extraction can yield values differing by about two orders of magnitude—a factor of 100. Published concentrations therefore cannot simply be compared across studies without considering the assay.
Circulating arginine vasopressin is unstable and difficult to measure directly. Researchers and specialists often use copeptin, a more stable fragment released with vasopressin, as a surrogate. Copeptin has a role in specific clinical investigations of water-balance disorders, including the differential diagnosis of excessive thirst and urination. That is different from screening everyone with a general wellness panel.
The papers on measuring oxytocin and vasopressin and copeptin in fluid homeostasis disorders explain these methodological and clinical distinctions.
Why nonapeptides matter for health
The examples here span two broad areas: hormone and nervous-system biology involving oxytocin and vasopressin, and cosmetic skin biology involving Nonapeptide-1. The oxytocin–vasopressin systems participate in cardiovascular regulation, kidney water balance, stress responses, and social behavior. Related laboratory values may provide clinical context, but they do not directly measure these peptide signals.
Related nine-residue structures have been conserved across vertebrate evolution, reflecting the physiological importance of the family. Arginine vasotocin’s regulation of the bird kidney is one example.
For cosmetic Nonapeptide-1 research, the melanocortin pathway, skin inflammation, and possible nutritional influences are relevant biological context. Alpha-MSH itself contains 13 amino acids. It is the upstream signal with which Nonapeptide-1 is proposed to compete. Keratinocytes release alpha-MSH after ultraviolet exposure, providing a rationale for investigating this pathway.
By contrast, high-sensitivity CRP measures systemic inflammation. It offers, at most, broad background; it does not measure melanocortin signaling or alpha-MSH activity. White blood cell counts and other inflammatory markers likewise do not directly capture the specific overlap between innate immunity and melanocortin biology.
Which biomarkers relate to nonapeptide biology?
Hormone physiology and cosmetic skin research involve different questions. Depending on the clinical situation, the following markers can help explain the background. None measures a general “nonapeptide activity,” and none establishes that a cosmetic peptide works.
- Plasma sodium helps clinicians assess water balance, which AVP helps regulate. Vasopressin affects kidney water reabsorption, while serum sodium reflects the combined result of several regulatory processes. Low sodium, or hyponatremia, and high sodium, or hypernatremia, can indicate disturbed water balance. Sodium is not a direct AVP test.
- Morning cortisol can inform specific questions about the hypothalamic–pituitary–adrenal, or HPA, axis. It is not a general stress score or an oxytocin test. A meta-analysis of intranasal oxytocin during laboratory tasks found no significant overall effect on cortisol. Reductions appeared mainly when HPA activation was strong and in clinical populations.
- High-sensitivity CRP is a systemic inflammation marker. It does not establish the effects of Nonapeptide-1 or provide a reason for routine blood testing before cosmetic use.
- White blood cells and a differential blood count can help assess immune-cell patterns in an appropriate clinical investigation. They do not directly measure nonapeptide signaling.
- Ferritin reflects iron stores. Iron deficiency can cause pallor, a change in appearance distinct from melanin-related pigmentation. When clinically indicated, assessing iron status may help explain a skin-tone change without attributing everything to MC1R signaling.
- Vitamin B12 may be relevant when deficiency is suspected. A case report links B12 deficiency with hyperpigmentation. Testing vitamin B12 can then help investigate pigmentation changes unrelated to UV exposure. Cosmetics targeting MC1R do not correct an underlying deficiency.
- Prolactin supports milk production, while oxytocin supports milk ejection. The hormones interact in lactation, but a prolactin level is not an oxytocin measurement. Their relationships with social behavior are also being investigated.
Laboratory results support clinical assessment; alone, they do not establish a diagnosis. This list is not a routine testing panel for people interested in cosmetic peptides.
Injectable oxytocin, injectable vasopressin, and the 12-amino-acid analog terlipressin are prescription medicines administered under medical care. Compounded nasal oxytocin products are not FDA-approved. The experimental applications described here are not recommendations for self-treatment.
Frequently asked questions
What is a nonapeptide?
A nonapeptide is a peptide containing nine amino acid residues. Its linear backbone has eight peptide bonds. In cyclic examples such as oxytocin and vasopressin, an additional disulfide bridge closes a ring. Nonapeptides fall within the oligopeptide size category.
Classic biological examples include oxytocin and arginine vasopressin (AVP), both cyclic nine-residue peptides produced in the hypothalamus. Nonapeptide-1 is a synthetic cosmetic nonapeptide investigated for skin-brightening applications.
What are examples of nonapeptides?
Oxytocin has the cyclic sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂. It has an established role in childbirth and participates in social bonding. Calling it simply the “bonding hormone” leaves out much of its complex biology.
Arginine vasopressin has the cyclic sequence Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH₂. It regulates kidney water reabsorption and vascular tone—the degree of tension in blood vessel walls.
Arginine vasotocin and mesotocin have corresponding roles in many nonmammalian vertebrates, while isotocin is found in teleost fish. These molecules are also cyclic nonapeptides.
Nonapeptide-1 is a synthetic cosmetic example studied as an MC1R-pathway modulator for brightening skin. Bradykinin is another human nonapeptide; unlike oxytocin and vasopressin, it is linear.
How does a nonapeptide differ from a decapeptide?
A nonapeptide has nine amino acids and eight peptide bonds in its linear backbone. A decapeptide has ten amino acids and nine corresponding bonds.
For the examples discussed here, shape is especially important. Oxytocin and vasopressin are cyclic, whereas cosmetic Decapeptide-12 is linear. Their Cys1–Cys6 disulfide bridge creates a ring whose geometry matters for receptor binding. Amino acid count alone does not tell you whether a peptide is linear or cyclic.
Are nonapeptides found in food?
Nine-residue sequences can form when proteins break down, including during digestion of casein, whey, and fish proteins. That does not establish how much of a particular nonapeptide reaches the bloodstream intact.
The PEPT1 transporter mainly carries di- and tripeptides—chains of two or three amino acids. Larger oligopeptides need other uptake routes. Inflammatory signals can affect transport: in cultured human intestinal cell layers, interferon-gamma increased PEPT1 expression and dipeptide transport.
Nine-residue nonapeptides are outside PEPT1’s well-characterized di- and tripeptide specificity. The cited cell study does not quantify intact nonapeptide absorption from food in humans, so it cannot supply a general absorption percentage.
Are nonapeptides safe?
Safety cannot be answered for an entire size category. Oxytocin and vasopressin occur naturally and have well-described physiological roles, but administering additional amounts is not automatically safe regardless of dose or circumstances.
The FDA has approved oxytocin for obstetric and postpartum uses, vasopressin for vasodilatory shock, and the related 12-residue analog terlipressin for hepatorenal syndrome. Each has its own prescribing information and risk profile.
Oxytocin’s prescribing information contains a prominent notice that it is not indicated for purely elective labor induction without a medical reason. Terlipressin carries a boxed warning for serious or fatal respiratory failure. These warnings should not be treated as interchangeable.
All three are prescription medicines used in obstetric, hospital, or intensive-care settings. They are not intended for unsupervised use. The treating clinician determines whether a specific medicine is appropriate.
Synthetic cosmetic nonapeptides such as Nonapeptide-1 fall under US cosmetics law. Cosmetics generally do not require FDA approval before marketing, although color additives have separate approval requirements. Manufacturers are responsible for substantiating safety. Tolerability depends on the formulation, concentration, and individual skin characteristics.
What do nonapeptides do in skin care?
This question usually refers to Nonapeptide-1. The synthetic cosmetic ingredient is intended to reduce melanin synthesis by competing with alpha-MSH at MC1R.
The hypothesis is that receptor competition weakens downstream signals involved in tyrosinase activation and melanin production. Clinical evidence in humans specifically for Nonapeptide-1 remains limited. A plausible mechanism still needs to translate into a demonstrated visible effect.
Under FDA cosmetics law, a cosmetic can be described as supporting the appearance of a more even skin tone. Claiming that it treats a pigmentation disorder goes beyond an appearance-only claim and changes the regulatory question.
How many amino acids does a nonapeptide have?
A nonapeptide has nine amino acids, joined by eight peptide bonds in a linear backbone.
In oxytocin and vasopressin, an intramolecular disulfide bond between two cysteines closes a ring. The backbone still has eight peptide bonds. The additional covalent bridge restricts flexibility and changes the molecule’s three-dimensional shape. Other ways of making cyclic peptides exist; this description concerns these two hormones.
Is a nonapeptide an oligopeptide?
Yes. “Oligopeptide” commonly describes a short chain of roughly 2–20 amino acids, although conventions vary. Nine residues fall comfortably within that range.
Oxytocin and vasopressin are therefore structurally oligopeptides as well as nonapeptides. Over evolution, they have taken on central roles as hormonal and neural signaling molecules.
References and sources
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Tom N, Assinder SJ. Oxytocin in health and disease. Int J Biochem Cell Biol. 2010;42(2):202-5.
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National Center for Biotechnology Information. PubChem Compound Summary for CID 439201, Bradykinin.
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National Center for Biotechnology Information. PubChem Compound Summary for CID 439302, Oxytocin.
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US Food and Drug Administration. Understanding the Risks of Compounded Drugs.
Further background reading: