Nasal sprays.
Direct to bloodstream.

Swallow a peptide and it is gone. Gastric acid and intestinal proteases break the chain apart before it reaches circulation, which is why almost every peptide in research use is injected. The nasal cavity offers a third option, and the reason has more to do with anatomy than chemistry.

Nasal mucosa is thin, densely vascularized, and drains directly into systemic circulation without passing through the liver. That alone removes first-pass metabolism. The more interesting property is what sits at the roof of the cavity: the olfactory epithelium is the only place in the body where neurons are exposed to the outside environment. Along the olfactory and trigeminal nerve pathways, a molecule can reach the central nervous system without crossing the blood-brain barrier at all^[1].

This is the reason a nasal dose of a CNS-targeted peptide is not simply a needle-free version of the same injection. The two routes deliver to different compartments. Semax and Selank are dosed intranasally in Russian clinical practice specifically because the target is central, not because injections were inconvenient.

The limits are real, though. Absorption falls off sharply with molecular size, roughly past 6 kDa, and hydrophobic peptides cross the mucosa poorly regardless of size. Formulation work in this area has focused on permeation enhancers for exactly that reason: bile salts and glycofurol measurably raise nasal bioavailability of peptides that would otherwise absorb badly^[2]. A large peptide in a spray bottle without an enhancer system is mostly an expensive way to irrigate a sinus.

Two practical consequences follow. Dose volume is capped by the cavity itself, generally around 100 to 150 microliters per nostril before the excess simply runs off or is swallowed, which is why nasal formulations run at higher concentrations than injectables. And mucociliary clearance sweeps the cavity continuously, giving any molecule a limited residence window to be absorbed before it is moved toward the throat.