Capsules, Injectables and Nasal Sprays: The Formulation Science of Peptide Delivery Formats

Published October 2026 · Patriot Labs Research Team · ~17 min read

The short answer

Capsules, injectables and nasal sprays are three formulation formats that differ in the biological barrier a peptide must cross, not usually in the peptide itself: a capsule must survive the digestive tract, an injectable bypasses it, and a nasal spray crosses the lining of the nose. Because the gut is built to digest peptides, oral bioavailability is typically under one percent, which is why injectables — reconstituted from a lyophilized powder — remain the default format and nasal delivery occupies a limited middle ground.

Everything else in this guide follows from that single fact: a peptide is a chain of amino acids, and the body has dedicated machinery for taking such chains apart. Each delivery format is an engineering answer to the question of how to get an intact peptide from outside the body to its target before that machinery destroys it. The oral route faces the harshest barrier and needs the most help; injection avoids the barrier at the cost of being an injection; the nose offers a shortcut that works only for the right molecules in the right formulation. The sections below take each in turn, then set them side by side.

The physical forms a peptide takes

Before comparing routes, it helps to separate the physical form of a peptide from the route it is formulated for, because the two are often confused. A research peptide almost always starts as a lyophilized (freeze-dried) powder, because that is its most stable state. To be studied it is reconstituted into a solution. That solution is what fills an injectable format; a different solution, with its own enhancers and preservatives, is what fills a nasal spray; and an oral format wraps the peptide, usually with protective excipients, inside a capsule or tablet.

So the three formats are not three different molecules. They are three different packages built around the same kind of fragile cargo, each designed for the specific gauntlet that package will run. The rest of this guide is about those gauntlets: what each format has to survive, and the formulation chemistry used to help it.

Why the gut degrades peptides: the oral barrier

The oral route is the one everyone wants — a capsule is painless and familiar — and it is the one peptides fail at most completely. The reason is that the gastrointestinal tract is, in effect, a peptide-destruction system, and it attacks on two fronts: enzymes that cut the chain, and a wall that blocks what is left.

The enzymatic front begins in the stomach, where acid and pepsin start cleaving peptide bonds, and continues in the small intestine, where the pancreatic proteases — trypsin, chymotrypsin, elastase and the carboxypeptidases — and roughly fifteen brush-border enzymes on the intestinal surface finish the job. The effect is dramatic: insulin, a well-studied example, is reported to be almost completely degraded by trypsin, chymotrypsin and elastase within an hour in that environment. On top of the enzymes sits a mucus gel, a hundred to a few hundred micrometres thick, that large molecules struggle to cross.

Any peptide that survives digestion then meets the absorption barrier: the epithelial lining of the gut. Here size and water-solubility work against it. Membrane permeability falls off sharply above roughly one kilodalton, and most peptides are hydrophilic — their log P is below zero — so they cannot partition into the lipid bilayer to cross through cells. The alternative, slipping between cells, is nearly closed too: those paracellular spaces are only a few ångström wide and make up well under one percent of the surface. The combined result is that oral bioavailability for peptides is typically below one percent, and sometimes below a tenth of that. For the degradation chemistry that governs this, see How Peptides Degrade.

Permeation-enhancer chemistry: coaxing peptides across

The gap between “under one percent” and “useful” is what permeation-enhancer chemistry tries to close, and the clearest worked example is oral semaglutide, the first orally formulated GLP-1 peptide. It is co-formulated with a permeation enhancer called SNAC (salcaprozate sodium), included in large excess relative to the peptide.

SNAC works by more than one mechanism. Its best-documented action is to raise the pH immediately around the dissolving tablet, which inactivates pepsin and protects the peptide from proteolysis in the stomach — protection reported to be effective at or above about pH 5. It is also thought to fluidize the cell membrane, and recent constant-pH molecular-dynamics modelling proposes that SNAC inserts into the membrane and forms transient, self-filling defects that a peptide can sink through while the membrane stays intact. A related enhancer, sodium caprate (C10), acts more on the junctions between cells; the two have been compared head to head as representative enhancer strategies. The honest bottom line is that even with SNAC, oral semaglutide's bioavailability is still only about one percent — the chemistry makes an unusable number barely usable, and it works only because the molecule is potent enough to matter at that level.

Other oral strategies tackle the same barrier from different angles: enteric coatings that keep the capsule closed through the acidic stomach and open at the higher pH of the intestine, enzyme inhibitors co-formulated to blunt the proteases (used sparingly because of toxicity), and particle- or nanoparticle-based carriers that shield the peptide and ferry it to the epithelium. All of them are fighting the same two-front battle described above, which is why oral peptide delivery remains one of the hardest problems in formulation.

Why injectables are the default format

Injection wins by refusing to fight. A subcutaneous or intramuscular injection places the peptide, already dissolved in solution, directly into tissue, so it never encounters stomach acid, pancreatic proteases, the mucus layer or the gut wall. The barrier that defeats the oral route simply is not in the path.

The consequence is high, reproducible systemic bioavailability, and that is the reason the injectable format — reconstituted from a lyophilized powder shortly before use — is the default for peptides in both approved pharmaceutical products and research settings. It is not that injection is pleasant; it is that it is the format in which a peptide reliably survives to reach the circulation. This is also why so much peptide chemistry is aimed not at the delivery format but at what happens after the peptide is in the blood, where plasma proteases and kidney clearance still remove it quickly. Modifications such as lipidation and PEGylation are designed to slow that removal, which is a different problem from getting the molecule in; see Peptide Half-Life Extension and What Is Lipidation? for that side of the story. The counterion and salt form a peptide is supplied in also affect how readily it dissolves for an injectable solution, covered in TFA vs Acetate in Peptides.

The nasal route: mucosal formulation science

The nose is a genuine shortcut for some peptides. The nasal cavity is lined with a thin, well-supplied mucosa, and a spray that deposits peptide solution onto it can send some of that peptide into the bloodstream while bypassing both stomach acid and the first-pass metabolism of the liver that a swallowed, absorbed peptide would face. For the right molecule, that is a real advantage over swallowing.

But the nasal route has hard physical limits that define its formulation science. Each nostril holds only a small volume of liquid, so only a small quantity can be delivered at once. The mucus blanket that protects the nasal lining is swept toward the throat and clears what is deposited within minutes, giving a short absorption window. Nasal enzymes degrade some of the peptide on contact, and absorption across the mucosa falls off for molecules much above a kilodalton. Formulators respond with permeation enhancers, mucoadhesive agents that make the formulation cling longer, and careful control of droplet size so the spray deposits in the nasal cavity rather than running off or reaching the lung. A number of peptides have been formulated as approved nasal products over the decades, which is the best evidence that the route works within its limits.

The pattern in that table is informative: the established nasal peptides are relatively small, which fits the molecular-size limit of mucosal absorption, and the newest entry, Baqsimi, is a dry powder rather than a solution — a reminder that even within one route the physical form keeps being re-engineered.

Why peptides are supplied as lyophilized powder

Underneath all three formats sits one more formulation decision: what state the peptide is stored in before it is ever put into a capsule, syringe or spray. The answer is almost always a lyophilized (freeze-dried) powder, because a peptide is far more stable dry than dissolved.

In solution, water is both the medium and the enemy. It drives the main degradation routes — hydrolysis of the backbone, oxidation of susceptible side chains, deamidation of asparagine and glutamine, and aggregation of unfolded chains — and all of them run faster as temperature rises. Freeze-drying removes that water, leaving a dry cake in which those reactions slow dramatically, so the peptide can be stored and shipped far longer than a ready-made solution would allow. The trade-off is that lyophilization is itself a delicate process — freezing and drying stresses can damage a fragile molecule unless the formulation includes the right stabilizers — which is why rational design of lyophilized formulations is a specialty of its own. The powder is then reconstituted into solution when it is needed. This is the reason research peptides are typically supplied lyophilized and reconstituted before study, and why cold-chain handling still matters even for the dry form. Patriot Labs supplies its research peptides as lyophilized material and reports each lot's identity and purity from an independent US laboratory by HPLC and mass spectrometry, with the certificate of analysis on the product page. For the details, see What Is Lyophilization?, How to Reconstitute Research Peptides, and Cold Chain for Research Peptides.

The three formats side by side

The table pulls the three formats together against the barrier each one has to beat. Read across, the pattern is consistent: the more of the body's defences a format bypasses, the more of the peptide survives.

This ranking is a statement about biology and formulation, not about any product. It explains why the overwhelming majority of peptides are studied as injectables, why a handful of small peptides have found a home in nasal sprays, and why an oral peptide is a notable feat of chemistry rather than a routine option.

What this guide does not establish

This guide is an explanation of pharmaceutical formulation science. It is not guidance on using anything, and several boundaries are worth stating plainly.

• Nothing here describes using any product by any route. Formats are described to explain the formulation chemistry and the biological barriers each one faces, not to suggest that any compound should be swallowed, injected, sprayed or otherwise administered. No format is recommended over another for any use.
• Bioavailability figures are pharmacokinetic facts, not instructions. Percentages and comparisons are drawn from the published pharmaceutical literature and describe how molecules behave in that research; they are not statements about any research peptide's safety, effectiveness or suitability for any purpose.
• The approved drug examples are illustrations, not endorsements. Oral semaglutide, desmopressin, calcitonin and the other named products are cited because their formulation science is documented; naming them says nothing about any Patriot Labs product, which is supplied for in-vitro research only.
• Nothing here describes where Patriot Labs’ peptides are synthesized. Patriot Labs is a first-responder-owned, US-based company, and the only quality facts used here are that its material is supplied lyophilized and that every lot is tested by an independent US laboratory by HPLC and mass spectrometry, with the COA available on the product page.

Frequently asked questions

What is the difference between peptide capsules, injections, and nasal sprays?

They are three formats that differ in the biological barrier the peptide has to cross, not usually in the peptide itself. A capsule has to survive the stomach and intestine, where acid and a battery of proteases digest peptides and the gut lining blocks large, water-loving molecules, so oral bioavailability for peptides is typically under one percent. An injectable places the peptide directly under the skin or into muscle from a reconstituted solution, bypassing the gut entirely, which is why most peptides are formulated this way. A nasal spray delivers a small volume of solution onto the nasal lining, avoiding stomach acid and first-pass liver metabolism but limited by how little liquid the nose holds and how quickly it clears. The format is a formulation choice driven by those barriers.

Why can't most peptides be taken as a capsule?

Because the gastrointestinal tract is built to break peptides down into amino acids. In the stomach, acid and pepsin begin cleaving the chain; in the small intestine, pancreatic proteases (trypsin, chymotrypsin, elastase and carboxypeptidases) and roughly fifteen brush-border enzymes continue the job, so insulin, for example, is almost completely degraded within an hour in that environment. Even a peptide that survives faces the gut wall: peptides above about one kilodalton cross cell membranes poorly, most are hydrophilic and cannot slip through the lipid bilayer, and the spaces between cells make up well under one percent of the surface and are only a few angstroms wide. The combined result is an oral bioavailability usually below one percent.

What is SNAC, and how does it help oral peptides like oral semaglutide?

SNAC (salcaprozate sodium) is a permeation enhancer co-formulated with the peptide in oral semaglutide (Rybelsus). It is included in large excess relative to the peptide. Its best-documented action is to raise the pH immediately around the dissolving tablet, which inactivates pepsin and shields the peptide from stomach proteases; it is also thought to fluidize the cell membrane and, in recent molecular-dynamics models, to form transient membrane defects that let the peptide slip across the gastric lining. Even with SNAC, oral semaglutide's bioavailability is still about one percent, which works only because the molecule is extremely potent.

Why are most research peptides injected?

Because injection sidesteps the barrier that defeats oral delivery. A subcutaneous or intramuscular injection places the peptide, dissolved in solution, directly into tissue, so it never meets stomach acid, pancreatic proteases or the gut wall, and a large fraction reaches the circulation. That high and reliable bioavailability is why the injectable route, reconstituted from a lyophilized powder, is the default format for peptides in both pharmaceutical products and research settings. The peptide still has to survive proteases and clearance in the blood, which is a separate problem addressed by half-life-extension chemistry rather than by the delivery format.

How do nasal peptide sprays work, and which peptides are given nasally?

A nasal spray deposits a small volume of peptide solution onto the mucosa lining the nasal cavity, where some of it can be absorbed into the bloodstream while avoiding stomach acid and first-pass liver metabolism. The route is constrained by biology: each nostril holds only a small volume, the mucus layer clears what is deposited within minutes, nasal enzymes degrade some of the peptide, and absorption falls off for molecules much above a kilodalton, so nasal formulations lean on permeation enhancers and mucoadhesives. Several peptides have been formulated as approved nasal products, including desmopressin, buserelin, nafarelin, salmon calcitonin and oxytocin.

Why are peptides supplied as lyophilized powder instead of a ready-made solution?

Because peptides are far more stable dry than dissolved. In solution a peptide is exposed to water-driven breakdown routes — hydrolysis of the backbone, oxidation of side chains, deamidation and aggregation — that slowly degrade it, and these run faster at higher temperatures. Freeze-drying (lyophilization) removes the water and leaves a dry cake that is chemically much more stable and can be stored longer, then reconstituted into solution when needed. This is why research peptides are typically shipped as a lyophilized powder and reconstituted before study, and why cold-chain handling still matters.

Which delivery format has the best bioavailability?

As a matter of pharmacokinetics, injection delivers by far the highest and most reproducible systemic bioavailability, because it bypasses the gut and first-pass metabolism; nasal delivery is intermediate and variable; and oral delivery is lowest, usually under one percent, which is why oral peptides must be either highly potent or paired with permeation enhancers. This ranking describes the formulation science and says nothing about how any compound should be used — it simply reflects which biological barriers each route avoids.

Does the delivery format change the peptide molecule itself?

Usually not the core sequence, but the format often dictates other molecular choices. The same peptide can be filled into a capsule, dissolved for injection or sprayed into the nose, and the amino-acid sequence is identical in each. What changes is the surrounding formulation — enhancers, buffers, stabilizers, enteric coatings — and sometimes the molecule is deliberately engineered for a route, for instance by adding a fatty-acid chain or other half-life-extension modification so an injectable lasts longer, or by choosing a counterion and salt form that dissolve well. The format and the molecule are designed together.

References

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2. Drucker, D. J. (2020). Advances in oral peptide therapeutics. Nature Reviews Drug Discovery, 19(4), 277–289. doi:10.1038/s41573-019-0053-0
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For related reading, see What Is Lyophilization? and How to Reconstitute Research Peptides for the dry-powder-to-solution step, Peptide Half-Life Extension for what happens to a peptide once it is in the blood, How Peptides Degrade for the chemistry of the breakdown routes, and Cold Chain for Research Peptides for handling.

All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human or veterinary consumption. This guide is educational and describes peptide delivery formats as pharmaceutical formulation science in general terms; it is not medical advice, does not describe how to use any product by any route, and makes no claim about the composition, format or suitability of any specific product.