Module 3 of 5

How They're Used

Administration routes, bioavailability, and the reconstitution calculator

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 1-3) 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 1-3) 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 temporarily raise stomach pH and enhance absorption. Even so, only ~1% of the dose reaches the bloodstream -- requiring a much higher oral dose than injectable.

Why Injection Works

Subcutaneous injection bypasses all three barriers. The peptide enters tissue directly, then slowly absorbs into the bloodstream through capillaries. This gives 50-80% bioavailability with predictable, consistent dosing.

The Future: Oral Peptides

Research is advancing rapidly on oral peptide delivery: enteric coatings, permeation enhancers, nanoparticle carriers, and micro-needle capsules (like Rani Therapeutics' RaniPill) may make needles obsolete within a decade.

The Reconstitution Lab

Injectable peptides come as a freeze-dried powder (lyophilized) that must be mixed with bacteriostatic water before use. This calculator helps determine the right concentration and syringe draw volume.

Calculator
Concentration
2.5 mg/mL
Draw Volume
10 units (0.10 mL)
5 mg lyophilized powder 10 units insulin syringe

Storage & Handling

Peptides are delicate molecules. Improper storage is one of the most common reasons for reduced effectiveness. Understanding degradation pathways helps explain why strict handling protocols exist.

Lyophilized
Freeze-dried powder (most stable) -- can be stored at room temperature for weeks or refrigerated for months. The removal of water prevents most degradation pathways. Always store in a cool, dark, dry place.
Reconstituted
Mixed with bacteriostatic water -- must be refrigerated at 2-8C (36-46F). Typically stable for 2-4 weeks refrigerated. Never freeze reconstituted peptides as ice crystals can denature the protein structure.
Degraded
What destroys peptides -- heat (above 25C accelerates breakdown), light (UV causes oxidation), agitation (shaking denatures structure), 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 denature the peptide at the air-water interface, reducing potency.
  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. Wang W et al. "Impact of injection sites on clinical pharmacokinetics of subcutaneously administered peptides and proteins." J Pharmacokinet Pharmacodyn. 2021;48(5):599-612. PMID 34186147
  4. Samaridou E, Alonso MJ. "Overview of intranasally delivered peptides." Expert Opin Drug Deliv. 2018;15(12):1215-1233. 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 35838946
  6. Bak A et al. "Physicochemical and Formulation Developability Assessment for Therapeutic Peptide Delivery." AAPS J. 2015;17(1):144-155. PMC4287299
  7. Jorgensen L et al. "Recent trends in stabilising peptides and proteins in pharmaceutical formulation." Expert Opin Drug Deliv. 2009;6(11):1219-30. PMC5665799
  8. Mapinhi BP, Castaneda YS. "Current Understanding of SNAC as an Absorption Enhancer." Crit Rev Ther Drug Carrier Syst. 2024;41(2):71-113. PMID 38230324

Knowledge Check

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Practice Exercises

Reinforce your understanding with interactive exercises.

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