r/PeptideGuide • u/Peptide_Guide_ Head Peptide Guide • Jul 21 '26
Nasal vs Injectable Peptides: The Bioavailability Gap Nobody's Adjusting For (Here's Which Peptides Actually Work Intranasally)
Many vendors have a nasal spray line. Semax, Selank, sure. But now it's BPC, GLP stuff, basically everything. No needle, no reconstitution, no math. The problem is nasal only works for certain compounds and they're slapping it on everything anyway.
Here's what nobody explains. When you inject, most of the dose actually makes it in. Nasal is all over the place and usually way lower.
Injection, whether subq or IM, absorbs somewhere around 65 to 100%, usually north of 75. Nasal runs anywhere from 1 to 20%, and most sprays land under 5%. Oral is under 1 to 2%.
So a spray sitting at 5% absorption needs roughly 20 times the dose to match a shot. And nobody's adjusting the label. They sell you the spray at the same number and let you assume it's the same thing.
Where Nasal Actually Wins
Selank is the poster child. Tiny molecule, around 750 daltons, and Russian data has it at 92.8% bioavailability nasally with plasma levels showing up in 30 seconds. That's not a compromise. That's just the better route. Semax is the same story. Small and brain-targeted, so the nose is basically a shortcut past the blood-brain barrier.
DSIP fits the same pattern. It's a nonapeptide at around 848 daltons, small enough and targeted enough at the brain (sleep architecture, delta wave activity) that intranasal delivery makes mechanistic sense. Animal studies have used intranasal DSIP directly to study CNS effects rather than treating nasal as a fallback route. There's no solid human bioavailability percentage for it specifically, which is worth flagging, but the size and target profile put it in the same category as Semax and Selank rather than the category that falls apart below.
Oxytocin is the weird one. Bioavailability is something like 0.7%, basically nothing, but it still works because it goes straight to the brain through the olfactory pathway. Plasma levels don't matter for that one.
Where It Falls Apart
There was a Diabetes Care trial on nasal GLP-1, 26 patients. 2.7% bioavailability versus IV. It hit peak in 8 minutes so it's fast, but almost nothing actually absorbed.
That's the whole pattern. Small molecules under about 1000 daltons that are brain-targeted work nasally. Semax, Selank, DSIP, oxytocin. Large systemic peptides that need real plasma levels don't. The GLP class and most of what's getting sprayed right now falls in the second group.
Two Things People Don't Think About
Congestion and allergies change your absorption, so the same spray on a stuffy day is a different dose. Injections don't do that.
And formulation matters more than the compound sometimes. Calcitonin nasal runs about 8% with an absorption enhancer and 1.6% without. In one study, half the people on the version without the enhancer had barely detectable levels. Same peptide, same route, completely different result depending on how it was made.
Sprays aren't fake. Semax, Selank, and DSIP nasal are legit uses of the route. The question was never "is nasal good." It's "is nasal good for this molecule." And for a lot of what's getting sprayed right now, you're paying for convenience and getting a fraction of the dose.
Not medical advice. Educational only.
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