Every compound referenced on this page is discussed strictly for in vitro research and laboratory use. None are approved for human consumption, therapeutic use, or veterinary application. Information on administration routes is provided as pharmacological reference, not as instructions for use in humans or animals.
The Concept: Bioavailability
Before the six routes, one idea: bioavailability. Bioavailability is the fraction of an administered dose that reaches systemic circulation in active form. It is the bridge between "how much you gave" and "how much actually arrived."
A peptide with 90% bioavailability via a given route delivers 0.9 milligrams of every 1.0 mg administered to circulation. A peptide with 1% bioavailability via a different route delivers 0.01 milligrams of the same 1.0 mg. That hundred-fold difference is not theoretical — it is, in practical terms, the entire difference between subcutaneous injection and oral administration for most peptides.
This is why route is not a minor detail. Route is the single biggest determinant of how much of a peptide actually does anything once given. A page comparing administration methods is really a page comparing bioavailability.
Side-by-Side Comparison
The six routes used for research peptides, compared on the attributes that drive the choice between them.
| Route | Typical Bioavailability | Onset | Equipment | Use Cases |
|---|---|---|---|---|
| Subcutaneous (subQ) | ~80–100% | Minutes–hours | U-100 insulin syringe | Default route — most research peptides |
| Intramuscular (IM) | ~80–100% | Faster than subQ | Longer needle (22–25 G) | Some peptides; faster onset |
| Intranasal | ~1–30% | Minutes | Nasal spray bottle | Nootropic peptides (Semax, Selank); BBB-targeting |
| Sublingual | ~3–10% | Minutes | Drops / lozenge | Limited; some short peptides |
| Topical | <1% systemic | Slow / local | Cream, gel, or serum | Local skin effects (GHK-Cu) |
| Oral | <1% (specialized: ~1%) | Slowest | Capsule or tablet | Only specifically engineered peptides |
Bioavailability at a glance
Bars show approximate ranges, not exact values. Bioavailability varies substantially by peptide within each route. The order of magnitude — high for injections, much lower for everything else — is the point.
Why Most Peptides Aren't Oral
The single most asked question about peptide administration is some version of "why can't I just take it as a pill?" The answer explains the structure of the entire field.
The digestive system is, fundamentally, a machine designed to break down peptides. That is its job. When you eat a steak, your body is breaking apart the same kind of peptide bonds that hold a research peptide together. Three obstacles do the work:
- Stomach acid (pH ~1.5–3.5). The low pH of the stomach denatures and hydrolyzes peptide bonds — the same hydrolysis that limits reconstituted peptide stability in water, accelerated dramatically by acid.
- Digestive enzymes. Pepsin in the stomach, trypsin and chymotrypsin in the small intestine — these enzymes exist specifically to cleave peptide bonds. They do not distinguish between food protein and a research peptide.
- First-pass metabolism. Anything that survives digestion is absorbed into the portal vein and routed through the liver before reaching general circulation. The liver further metabolizes most peptides aggressively.
The result is that oral bioavailability for unprotected peptides is typically well under 1% — not "much lower than injection," but "essentially zero." Most of the dose never makes it past the stomach. Of the fraction that does, most is broken down in the intestine. Of the fraction that survives that, the liver clears most of what remains.
The exceptions that exist
A small number of peptides have been engineered specifically for oral delivery. The most notable is oral semaglutide, which uses a permeation enhancer called SNAC (sodium N-(8-[2-hydroxybenzoyl] amino) caprylate) that temporarily protects the peptide and helps it cross the stomach lining. Even with this engineering, oral semaglutide bioavailability is approximately 1% — which is why the oral dose is around 100-fold higher than the equivalent subcutaneous dose. The route works, but only because the chemistry is built around it.
For peptides not specifically designed for oral use — which is almost all of them, including everything in the standard Peptide Database — oral is not a viable route. Capsule formulations of research peptides sold for "oral" administration are mostly absorbed in the mouth or sublingually rather than in the digestive tract, and bioavailability remains low.
Subcutaneous Injection (subQ)
Injection into the subcutaneous tissue — the fatty layer between the skin and the underlying muscle. A short, fine needle (typically 4–8 mm, 29–31 gauge) deposits the peptide solution where it is gradually absorbed into circulation through local capillaries and lymphatic vessels.
Subcutaneous is the default route for research peptides for a stack of reasons that combine into a clear winner: high and predictable bioavailability (80–100% for most peptides), simple equipment that costs cents per injection, minimal discomfort with proper technique, and a deep literature base — most published peptide pharmacokinetic data is in subQ-administered models.
The mechanics are also forgiving. The subcutaneous space is shallow enough that needle length and angle have wide tolerance, and the local fat tissue acts as a brief depot, smoothing absorption into circulation. Compared to the alternatives, subQ is the route with the fewest things that can go wrong.
Common subcutaneous injection sites
- Abdomen — the most common site in published protocols; absorption is reliable and consistent. Typically a few inches away from the navel.
- Thigh (front or outer) — straightforward access; slightly slower absorption than abdomen.
- Upper arm (back, posterior) — used for variety; requires another person or careful self-administration.
- Site rotation — for ongoing protocols, rotating sites avoids local tissue stress.
Insulin syringes are the standard tool
The U-100 insulin syringe — calibrated so that 100 units equals 1 mL — is the standard for subQ peptide administration. Its short needle, fine gauge, and unit-based markings match the typical volumes and doses of research peptides almost perfectly. The reconstitution math on every peptide page in this hub outputs "units on a U-100 syringe" because that is the tool the math is for. See the Reconstitution Reference for the conversion details.
Intramuscular Injection (IM)
Injection deeper into muscle tissue, beneath the subcutaneous layer. Requires a longer needle (typically 1–1.5 inches) and is performed at a perpendicular angle to the skin. Bioavailability matches subQ, but absorption is faster because muscle tissue is more vascularized.
Intramuscular delivery yields comparable bioavailability to subcutaneous for most peptides but is rarely the first choice in research contexts. It requires a longer needle, is more uncomfortable, has a higher technical bar, and the faster onset rarely matters for the time-courses of most peptide research. SubQ wins by default for peptides where either route would work.
The cases where IM is genuinely preferred: peptides with poor subcutaneous absorption (uncommon among research peptides), situations where a faster peak is specifically desirable, or larger-volume administrations that exceed practical subQ volumes. For most peptides indexed in this hub, IM is an option that exists but is not the default.
Intranasal
Administration as a fine spray or droplet into the nasal cavity, where the highly vascularized nasal mucosa absorbs the peptide directly into circulation. Some peptides also reach the central nervous system through the olfactory pathway, bypassing the blood-brain barrier.
Intranasal is the defining route for the nootropic peptide class. Semax and Selank were specifically developed for nasal delivery — their seven-residue structures and Pro-Gly-Pro stabilizing motif tolerate mucosal absorption, and the route provides a fast, needle-free delivery that suits how these peptides are studied. DSIP and a handful of other neuroactive peptides also use this route.
Two things make intranasal interesting beyond convenience. First, bioavailability is meaningfully higher than oral — typically 1–30% depending on peptide and formulation — because the nasal mucosa is vascular and lacks the digestive enzymes of the gut. Second, and more importantly, the olfactory pathway allows some compounds to reach the central nervous system directly, partially bypassing the blood-brain barrier. For peptides whose target is in the brain, that is a meaningful advantage.
The constraint is that intranasal works only for peptides small and stable enough to absorb through the nasal mucosa intact. Large or fragile peptides are not nasal candidates. The route is a specialty tool, not a general-purpose alternative to injection.
Oral
Swallowed administration, where the peptide must survive stomach acid, digestive enzymes, intestinal absorption, and hepatic first-pass metabolism. For unprotected peptides, the result is bioavailability so low the route is effectively non-functional.
The "Why Most Peptides Aren't Oral" section above covers the chemistry. For practical purposes: oral administration is not a route for peptides that were not specifically engineered for it. The one well-documented exception is oral semaglutide, where the SNAC formulation produces approximately 1% bioavailability — enough to be therapeutically useful when the oral dose is scaled up roughly 100-fold compared to the subcutaneous form, but still two orders of magnitude below injection.
A note on the broader category of "non-peptide research compounds" indexed in the Peptide Database — MK-677, 5-Amino-1MQ, NAD+, Methylene Blue. These are small molecules, not peptides, and the chemistry that makes peptides orally inviable does not apply to them. MK-677 in particular is reliably orally active because it is structurally a small molecule that resists digestion. When a "peptide" product is labeled for oral use and the bioavailability is actually reasonable, check whether it is genuinely a peptide.
Topical
Application of a peptide-containing preparation to the skin. The target is local tissue effect — skin remodeling, healing, anti-inflammatory action — rather than systemic delivery, because intact skin is an excellent barrier and systemic absorption is minimal.
Skin is engineered, evolutionarily, as a barrier. That is fundamentally incompatible with using it as a route for systemic peptide delivery. For most peptides, topical application delivers under 1% to circulation. The route's place is not in systemic dosing — it is in local effects on the skin itself.
The clearest example is GHK-Cu, the copper tripeptide, used in cosmetic and dermatological research for effects on skin remodeling, collagen synthesis, and hair follicle activity. Topical GHK-Cu acts on the tissue it touches; minimal systemic absorption is not a flaw, it is the design. The peptide's target is exactly where it is applied. Other healing peptides studied in topical contexts — small peptides like KPV or fragments of larger healing peptides — follow the same pattern.
Sublingual
Administration under the tongue, where the peptide is absorbed through the vascular oral mucosa directly into circulation, bypassing the digestive tract and the liver's first-pass metabolism.
Sublingual occupies a middle ground. By absorbing through the oral mucosa rather than passing through digestion, it avoids the catastrophic bioavailability loss of the oral route. By relying on diffusion across a tissue not designed for peptide absorption, it cannot match injection. Bioavailability typically falls in the 3–10% range — meaningfully better than oral, meaningfully worse than subQ.
The route works best for short, small peptides — the same class that tends to work intranasally. Larger peptides do not absorb sublingually in usable quantities. The route is occasionally seen in research contexts for specific compounds but is not a general-purpose alternative to injection. When a peptide product is described as "sublingual," consider whether the bioavailability assumption matches the labeled dose.
How to Read a Peptide's Recommended Route
Most peptide documentation specifies a route — and once you understand the bioavailability framework, those route recommendations become readable rather than arbitrary.
- "Subcutaneous" means the documented bioavailability and dosing assumes subQ delivery. Almost every peptide in this hub falls here.
- "Intramuscular" implies either a peptide with poor subQ absorption or a context where faster onset matters. Rare in research peptide work.
- "Intranasal" means the peptide was specifically developed or formulated for nasal delivery — typically a small nootropic peptide. Don't substitute injection without recalculating dose, because the bioavailability assumptions are different.
- "Oral" demands scrutiny. For a true peptide, this means either specialized engineering (rare) or that the documented bioavailability is very low and the dose has been scaled accordingly. For a non-peptide research compound (MK-677, methylene blue, etc.), oral is genuinely fine.
- "Topical" means the target is local tissue, not systemic exposure. The dose, frequency, and concentration are calibrated for skin contact, not for what enters circulation.
- No route specified — assume subcutaneous as the default, since this is the route most peptide pharmacokinetic data is derived from.
Frequently Asked Questions
What is the most common route for peptide administration?
Subcutaneous injection is the most common route for research peptide administration. It delivers high and predictable bioavailability — typically 80 to 100 percent for most peptides — uses simple equipment (a U-100 insulin syringe), and is the route in which most published peptide research is conducted. The vast majority of peptides in the NuGenia Peptide Database are administered subcutaneously.
Why can't most peptides be taken orally?
Oral peptide bioavailability is typically less than 1%. Peptides are large molecules that the digestive system treats as food — they are broken down by stomach acid and digestive enzymes before they can be absorbed intact. The few peptides engineered for oral use, such as the SNAC-formulated oral semaglutide, are structurally protected or chemically delivered to bypass digestion. For nearly every other research peptide, oral administration is not viable.
What does bioavailability mean for peptides?
Bioavailability is the fraction of an administered dose that reaches systemic circulation in active form. A peptide with 90 percent subcutaneous bioavailability delivers 0.9 of every administered milligram to circulation; a peptide with 1 percent oral bioavailability delivers 0.01. Route of administration is the single biggest determinant of bioavailability for any given peptide.
What is the difference between subcutaneous and intramuscular injection?
Subcutaneous injection delivers the peptide into the fatty layer just under the skin, using a short needle (typically 4 to 8 millimeters). Intramuscular injection delivers it deeper, into muscle tissue, using a longer needle. Both yield high bioavailability for most peptides. Subcutaneous is preferred for its simplicity and lower discomfort; intramuscular is sometimes used for peptides with poor subQ absorption or where a faster onset is needed because muscle tissue has more blood flow.
How does intranasal peptide administration work?
Intranasal administration delivers a peptide as a solution sprayed into the nasal cavity, where the highly vascularized nasal mucosa absorbs it into circulation. Bioavailability is typically 1 to 30 percent depending on the peptide. Intranasal is the route of choice for the nootropic peptides Semax and Selank, which were specifically developed for nasal delivery, and it allows some compounds to bypass the blood-brain barrier and act directly on the central nervous system.
Can peptides be applied to the skin?
Topical application is used for peptides with local effects, such as GHK-Cu and certain healing peptides in cosmetic research contexts. Systemic absorption through intact skin is generally very low — under 1% for most peptides — because skin is engineered as a barrier. Topical administration is appropriate when the target tissue is the skin itself, not when systemic exposure is required.
If I switch a peptide from subcutaneous to oral, do I use the same dose?
No. For most peptides, oral bioavailability is roughly 100-fold lower than subcutaneous. Matching milligrams across routes delivers a fraction of the effective dose. Route changes require recalculating dose using the bioavailability of the new route — and for most peptides, oral is not a viable route at all, regardless of dose scaling.
Which is better for nootropic peptides — injection or nasal spray?
For peptides like Semax and Selank, intranasal is the route they were developed for, and the existing research dose conventions assume nasal delivery. Injection bypasses the nasal mucosa but does not access the olfactory pathway that allows some compounds to reach the central nervous system directly. The two routes are not interchangeable, and dose conventions for these peptides are documented for the intranasal route specifically.
All compounds referenced on this page are discussed strictly for in vitro research and laboratory use. None are approved by the FDA for human consumption, therapeutic use, or veterinary application. This reference is provided for educational and reference purposes only and does not constitute medical advice. Information about administration routes is provided as pharmacological reference, not as guidance for administration in humans or animals.
Bioavailability ranges are approximate, aggregated from published pharmacokinetic literature, and vary substantially by peptide, formulation, and study conditions. For any specific peptide, confirm route and dosing assumptions against the certificate of analysis and primary research literature.