Nasal Delivery, and the Numbers Nobody Measured
Route-of-administration guides for peptides circulate with impressive precision — bioavailability percentages, dose multipliers, brain-level comparisons. Most of those numbers have no study behind them.
The appeal of a nasal spray over a needle is obvious, and the question behind it is legitimate: does the nasal route actually work for these compounds?
The honest answer has three parts. Nasal delivery of peptides is real and there are approved drugs that use it. The nose-to-brain pathway is anatomically real but its magnitude in humans is contested. And the specific figures circulating in this space — 30-40% bioavailability, requires 2-3× the injectable dose, higher brain levels than IV — are, for most of the compounds they are attached to, numbers with no published human study behind them.
That last part is the important one, because a fabricated number is more persuasive than an honest uncertainty.
What the nasal mucosa can and cannot do
The nose absorbs small lipophilic molecules well. Peptides are neither small nor lipophilic, and three things work against them:
- Surface area. The nasal cavity offers on the order of 150 cm², against roughly 100 m² for the small intestine.
- Mucociliary clearance. Whatever you spray is swept toward the pharynx and swallowed within roughly 15-20 minutes. That is the entire absorption window.
- Enzymes. Nasal mucosa contains peptidases. The route is gentler than the gut, not exempt from it.
The consequence is a steep relationship between molecular size and absorption. Small peptides can achieve usable nasal bioavailability. Larger ones generally cannot without absorption enhancers, and enhancers bring their own tolerability problems.
The reference class worth knowing
Approved nasal peptide drugs exist, and they are the right comparison for any claim in this area: desmopressin, calcitonin, nafarelin, buserelin.
Two features are worth noting. They are all small. And their nasal bioavailability is still low — frequently in the low single digits as a percentage of the injected dose.
The nose-to-brain question
There is a genuine hypothesis here. The olfactory and trigeminal nerves provide anatomical routes from the nasal cavity toward the CNS that bypass the blood-brain barrier, and animal studies support some direct transport.
In humans the magnitude is much less settled. The studies most often cited are small, and measuring brain concentrations non-invasively in people is hard, which is precisely why the question stays open. Plausible mechanism, weak human quantification.
That is a reasonable thing to find interesting. It is not a basis for stating that a given compound reaches the brain better nasally than intravenously.
Claims worth being specific about
Oxytocin. Intranasal oxytocin genuinely has hundreds of published trials — but the summary "confirming effectiveness" inverts where that literature ended up. Many early social-cognition findings failed to replicate, effect sizes shrank under larger and better-controlled designs, and whether behaviourally meaningful quantities reach the human brain remains actively debated. It is one of psychopharmacology's better-known replication problems, not a settled success.
Also worth correcting: there is no currently marketed FDA-approved intranasal oxytocin in the United States. Approved oxytocin is the injectable, used in obstetrics. Nasal preparations in research settings are compounded or investigational.
Bioavailability percentages and dose multipliers for unapproved compounds. A figure like "30-40% of injected" or "2-3× the dose" is a pharmacokinetic result. Producing one requires dosing humans by both routes and measuring plasma concentrations over time. For most compounds in this category those studies have not been done and not been published.
When a precise-looking number exists without a study behind it, the precision is the tell.
Claims of superiority over intravenous delivery. Beating IV means beating 100% bioavailability by definition of the comparison. That is an extraordinary claim for a route with a 20-minute clearance window, and it needs an extraordinary source — a human study, named, that you can read.
One correction that is safety-relevant
PT-141 (bremelanotide) was developed as a nasal spray, and the usual explanation for the switch — variable absorption between individuals — leaves out the part that matters. Blood pressure elevation was a documented problem during nasal development, and the product that reached approval in 2019 is the subcutaneous autoinjector, which carries blood pressure warnings of its own.
Framing that history as an absorption inconvenience removes the reason a regulator cared.
One compound where route is the wrong question
Melanotan II appears in these lists as a routing decision. It should not. Multiple national regulators have issued public warnings against it; it is unapproved everywhere; and reported harms include changing and atypical moles against a background concern about melanoma, along with case reports of rhabdomyolysis and priapism.
Whether it absorbs better nasally or subcutaneously is not the first question about it.
Where the list is right
The final section of the usual guide is sound. GLP-1 receptor agonists and growth hormone secretagogues are not viable nasally, and for the right reason: size and charge put them well outside what the nasal mucosa passes in useful quantity. Their approved and studied routes are subcutaneous — with the engineered exception of oral semaglutide, which needs an absorption enhancer and still lands near 1% bioavailability.
How to assess a nasal claim
- Is there a human pharmacokinetic study? Named, findable, with plasma concentrations. Not a forum consensus.
- Is there an approved nasal formulation of anything comparable? The approved list is short and the compounds on it are small.
- Does the molecular weight make it plausible? Large hydrophilic molecules do not become absorbable through enthusiasm.
- Is the number attached to a citation? Specific figures without sources are the most common form of invented data in this field.
None of this makes nasal delivery a dead end. It makes it a route with real constraints, a small set of genuine successes, and a marketing literature considerably more confident than its evidence.
Disclosure: Biohacker Life is published by the team behind Nalu Labs, which sells in this category. Weigh our coverage accordingly.
General information about pharmacokinetics and drug regulation. Not medical advice, and not a protocol. Nothing here is a recommendation to obtain or self-administer any compound by any route. Several substances named are prescription-only or unapproved, and legality varies by jurisdiction. If you think you are having a medical emergency, contact emergency services.
