Module 3 of 5

How They're Used

Administration routes, bioavailability, and the reconstitution calculator

This course is education, not medical advice.

How Are Peptides Taken?

Most peptides can't survive your stomach acid, which is why injection is the most common route. But newer technologies are opening doors to oral, nasal, and topical delivery.

Interactive

Click a route to learn more

Why can't you just swallow most peptides? Stomach acid (pH of about 1.5 to 2) denatures them, digestive enzymes cleave the peptide bonds, and the intestinal wall blocks large molecules from absorbing. Oral semaglutide (Rybelsus) is a rare exception that uses a special absorption enhancer called SNAC.

Understanding Bioavailability

Bioavailability is the percentage of a drug that actually reaches your bloodstream. It's one of the most important concepts in peptide therapy, and the reason most peptides need to be injected rather than swallowed.

What Destroys Peptides?

Three barriers work against oral peptides: stomach acid (pH of about 1.5 to 2) denatures the structure, digestive enzymes (pepsin, trypsin) cleave the peptide bonds, and the intestinal wall blocks large molecules from absorbing into blood.

The SNAC Breakthrough

Oral semaglutide (Rybelsus) uses SNAC (sodium N-[8-(2-hydroxybenzoyl)amino]caprylate) to buffer the pH right around the tablet and enhance absorption. Even so, only ~1% of the dose reaches the bloodstream.

Why Injection Works

Subcutaneous injection bypasses all three barriers. The peptide enters the tissue just under the skin (usually the abdomen, thigh, or upper arm), then absorbs into the bloodstream through capillaries. That is why most peptide drugs are injected.

The Future: Oral Peptides

Research is advancing on oral peptide delivery. Absorption enhancers like SNAC already let oral semaglutide be absorbed through the stomach wall, and other ways around the same three barriers are in development.

The Reconstitution Lab

Many injectable peptides, including the semaglutide vials bought online in the study in module 4, come as a freeze-dried powder (lyophilized) that must be mixed with bacteriostatic water before use. Approved pens such as Wegovy come ready to inject. This calculator shows the arithmetic that links the amount of water to the concentration and the syringe volume.

Educational math, not medical advice. This calculator only shows how water volume, concentration and syringe units relate. It is not dosing guidance and cannot tell you what dose, if any, is right for anyone. Most peptides sold online as research compounds are not approved for human use. Talk to a licensed clinician before using any peptide.
Calculator
Concentration
2.50 mg/mL
Draw volume (U-100 syringe: 100 units = 1 mL)
10.0 units (0.100 mL)
lyophilized powder 0 1 2 3 4 5 mL BAC water 2.0 mL

Storage & Handling

Peptides are delicate molecules. Temperature, shaking, and freeze-thaw cycles can all damage them, and understanding those degradation pathways helps explain why strict handling protocols exist.

Lyophilized
Freeze-dried powder (most stable). Removing the water shuts down most degradation pathways, which is why many peptides ship in this form. Store it in a cool, dark, dry place and follow the supplier's temperature guidance.
Reconstituted
Mixed with bacteriostatic water, a peptide in solution is far less stable than the powder. Keep it refrigerated, use it within the window your supplier gives, and do not freeze it.
Degraded
What destroys peptides: heat (oxytocin, for one, breaks down fast enough at elevated temperatures to need refrigeration), light (UV causes oxidation), agitation (shaking drives aggregation), and bacterial contamination. Signs of degradation include cloudiness, particles, or color change.
Never shake a reconstituted peptide. Gently swirl or roll the vial instead. Vigorous shaking creates foam and can make the peptide clump together at the air-water interface, reducing potency.

Common Questions About How Peptides Are Used

Short, sourced answers to the questions people ask most about how peptides are taken, why they're usually injected, and how to reconstitute and store them.

How are peptides administered?

Most peptides are administered by subcutaneous injection because they can't survive digestion. Injecting under the skin (usually the abdomen, thigh, or upper arm) skips the digestive barriers and allows self-administration. Other routes include oral (semaglutide with SNAC), nasal sprays for small peptides, topical creams that act locally, and intravenous delivery in clinical settings.

Why can't peptides be taken orally?

Three barriers destroy most oral peptides: stomach acid (pH of about 1.5 to 2) denatures their structure, digestive enzymes (pepsin, trypsin) cleave the peptide bonds, and the intestinal wall blocks large molecules from absorbing. Oral semaglutide (Rybelsus) is a rare exception using an absorption enhancer called SNAC, yet even then only about 1% of the dose reaches the bloodstream.

What is bioavailability in peptides?

Bioavailability is the percentage of a dose that actually reaches your bloodstream. It's a main reason most peptides are injected rather than swallowed: an injected dose skips the digestive barriers entirely, while oral peptides typically deliver under 1-2%.

How do you reconstitute a peptide?

Many injectable peptides come as a freeze-dried (lyophilized) powder that must be mixed with bacteriostatic water before use; approved pens come ready to inject. The amount of water sets the concentration (for example, 5 mg in 2 mL gives 2.5 mg/mL) which then determines the syringe draw volume. Gently swirl rather than shake the vial, since vigorous shaking can damage the peptide.

How should peptides be stored?

Lyophilized freeze-dried powder is the most stable form, because removing the water shuts down most degradation pathways; keep it in a cool, dark, dry place and follow the supplier's temperature guidance. Once reconstituted, a peptide is far less stable: keep it refrigerated, use it within the window your supplier gives, and never freeze it, since freeze-thaw cycles are a known stress on peptides.

  1. Renukuntla J et al. "Approaches for enhancing oral bioavailability of peptides and proteins." Int J Pharm. 2013;447(1-2):75-93. PMID 23428883
  2. Heda R et al. "Physiology, Pepsin." StatPearls. 2024. NBK537005
  3. Zou P, Wang F, Wang J, et al. "Impact of injection sites on clinical pharmacokinetics of subcutaneously administered peptides and proteins." J Control Release. 2021;336:310-321. PMID 34186147
  4. Al Bakri W, Donovan MD, Cueto M, et al. "Overview of intranasally delivered peptides: key considerations for pharmaceutical development." Expert Opin Drug Deliv. 2018;15(10):991-1005. PMID 30173579
  5. Bucheit JD et al. "Oral Semaglutide: A Review of the First Oral GLP-1 Receptor Agonist." Diabetes Technol Ther. 2020;22(1):10-18. PMID 31436480
  6. Bak A et al. "Physicochemical and Formulation Developability Assessment for Therapeutic Peptide Delivery." AAPS J. 2015;17(1):144-155. PMC4287299
  7. Zapadka KL, Becher FJ, Gomes Dos Santos AL, et al. "Factors affecting the physical stability (aggregation) of peptide therapeutics." Interface Focus. 2017;7(6):20170030. PMC5665799
  8. Solis-Herrera C, Kane MP, Triplitt C. "Current Understanding of Sodium N-(8-[2-Hydroxylbenzoyl] Amino) Caprylate (SNAC) as an Absorption Enhancer: The Oral Semaglutide Experience." Clin Diabetes. 2024;42(1):74-86. PMID 38230324
  9. Ashraf AR, Mackey TK, Vida RG, et al. "Multifactor Quality and Safety Analysis of Semaglutide Products Sold by Online Sellers Without a Prescription: Market Surveillance, Content Analysis, and Product Purchase Evaluation Study." J Med Internet Res. 2024;26:e65440. PMID 39509151
  10. Novo Nordisk. "WEGOVY (semaglutide) injection, US prescribing information." PDF as served on the fetch date. novo-pi.com wegovy.pdf
  11. Hornsby BD, Lee CH, Steele CA, et al. "Therapeutic peptides and proteins: Status and developments in drug delivery." J Control Release. 2026. PMID 41942053
  12. Yan J, Siwakoti P, Shaw S, et al. "Porous silicon and silica carriers for delivery of peptide therapeutics." Drug Deliv Transl Res. 2024. PMID 38819767

Knowledge Check

Test what you learned in this module.

Practice Exercises

Reinforce your understanding with interactive exercises.

Next Module

The Truth