How Intranasal Drug Delivery Reaches the Brain

Intranasal drug delivery uses the nose as a gateway to get medications into the bloodstream or directly into the brain, often within minutes. The approach has already produced several widely used products, from naloxone nasal spray for opioid overdoses to esketamine for treatment-resistant depression, and researchers are pushing it into territory that would have seemed far-fetched a generation ago: gene therapy, Alzheimer’s treatment, and mucosal vaccines. What makes the nasal route so appealing is a combination of rich blood supply, a thin tissue barrier, and a unique anatomical shortcut that lets certain drugs bypass the blood-brain barrier entirely.

How Drugs Travel From the Nose to the Brain

The inside of your nose is lined with two distinct types of tissue. Most of the nasal cavity is covered by respiratory epithelium, a mucous membrane packed with blood vessels that can absorb drugs into general circulation much like the lining of your mouth. But tucked up high in the nasal vault sits a smaller patch of olfactory epithelium, where nerve endings from the olfactory bulb of the brain extend dendrites directly into the nasal cavity. Gaps between these olfactory neurons and their supporting cells may allow molecules to slip through more easily than the tighter respiratory tissue below them.

This anatomy creates two direct routes from the nose to the brain. The olfactory pathway runs along olfactory nerve fibers through tiny perforations in the skull called the cribriform plate, landing molecules in the olfactory bulb and from there into deeper brain structures. The trigeminal pathway follows branches of the trigeminal nerve, which innervates much of the nasal lining and connects to the brainstem. Together, these routes let drugs reach the central nervous system without having to cross the blood-brain barrier, the notoriously selective membrane that blocks most drugs injected into a vein from entering the brain.¹ Animal studies have confirmed that intranasally delivered drugs can appear in the brain within minutes via both pathways.²

The Nose’s Built-In Defenses Against Drug Absorption

The nasal cavity was not designed for drug delivery. It was designed to warm, humidify, and filter inhaled air, and to keep foreign particles out. Two defenses in particular give drug developers headaches.

The first is mucociliary clearance. A layer of mucus coats the nasal lining, and tiny hair-like cilia beat in coordinated waves to push that mucus (and anything trapped in it) toward the throat. A drug sprayed into the nose typically has only about 15 to 20 minutes before clearance sweeps it away. To buy more time, formulators use mucoadhesive polymers, sticky materials that cling to the mucus layer and keep the drug in contact with the absorptive tissue longer.³

The second defense is enzymatic. The nasal mucosa contains a surprising arsenal of drug-metabolizing enzymes. In the olfactory region specifically, certain cytochrome P-450 enzyme activity is even higher than in the liver, driven by a three- to four-fold greater concentration of the electron-transfer protein those enzymes need to function.⁴ These enzymes evolved to neutralize inhaled toxins, but they also chew up drugs before they can be absorbed. Peptide-based drugs face an additional hurdle: the nasal lining is rich in peptidases and proteases that break down proteins. This is one reason delivering large therapeutic proteins through the nose remains difficult.

Naloxone and the Case for Rapid Systemic Delivery

Perhaps the most visible success story for intranasal delivery is naloxone, the opioid-overdose reversal agent. Speed matters enormously when someone stops breathing from an opioid overdose, and for years the standard approach was an intramuscular injection, which requires training and a willingness to use a needle. A nasal spray that a bystander can administer in seconds changed the equation.

In a Phase I study of healthy volunteers, intranasal naloxone at doses of 2 mg and 4 mg produced higher peak blood concentrations than the traditional 0.4 mg intramuscular injection. All three intranasal doses reached more than half their peak concentration within 10 minutes, with full peak levels arriving at 15 to 30 minutes. The nasal bioavailability was roughly half that of the intramuscular route, meaning you need a larger nasal dose to match a given injection, but the higher dosing more than compensates.⁵ A systematic review of naloxone pharmacokinetics confirmed that both the 2 mg and 4 mg approved nasal sprays achieve drug exposure exceeding that of the historic intramuscular benchmark, providing adequate levels for overdose reversal.⁶

Naloxone illustrates a broader principle: even when the goal is systemic drug delivery rather than brain targeting, the nose offers advantages for situations where speed and ease of use outweigh the efficiency of a needle. The nasal lining’s dense blood supply allows rapid absorption into general circulation, and the patient (or a bystander) does not need medical training to press a nozzle into a nostril.

Esketamine for Treatment-Resistant Depression

Intranasal esketamine (marketed as Spravato) became one of the first genuinely novel antidepressant mechanisms in decades when it received regulatory approval. Traditional antidepressants typically take weeks to show an effect. Esketamine works on the glutamate system rather than serotonin, and the nasal delivery route gets it to the brain quickly enough to produce measurable improvements within hours.

In a randomized trial of patients with treatment-resistant depression, all three tested doses of intranasal esketamine (28 mg, 56 mg, and 84 mg) outperformed placebo on the standard depression rating scale, with a clear dose-response relationship. The highest dose produced the largest improvement. The antidepressant response appeared rapidly and continued to grow with repeated dosing over an open-label follow-up period.⁷

A meta-analysis pooling data from multiple trials found that esketamine began outperforming placebo on depression scores as early as two to four hours after the first dose, and maintained that advantage through 28 days of double-blinded treatment. For patients with major depression and suicidal ideation, resolution of suicidal thinking was about twice as likely with esketamine as with placebo at the two-to-four-hour mark, though that difference faded by day 28.⁸ The speed of that initial response is a direct consequence of the nasal delivery route getting the drug into the brain faster than an oral pill ever could.

Oxytocin, Insulin, and Other Brain-Targeted Molecules

Esketamine is a small molecule that can also be given intravenously. More ambitious nose-to-brain work involves peptides and proteins that would never survive the digestive tract and struggle to cross the blood-brain barrier from the bloodstream. Oxytocin is the longest-studied example. Researchers have sprayed it into people’s noses for decades, originally to trigger milk letdown in breastfeeding mothers and more recently to study its effects on social behavior, trust, and emotional processing. There is active interest in using intranasal oxytocin to treat conditions marked by social dysfunction, including autism spectrum disorders and schizophrenia.⁹

A key uncertainty, though, is whether the behavioral effects seen after intranasal oxytocin actually depend on direct nose-to-brain transport or whether they arise from oxytocin entering general blood circulation and acting on the brain indirectly. Only recently have animal microdialysis studies begun to probe this question directly, and the evidence is still thin.¹⁰ This matters because if the nasal route is just a convenient way to get oxytocin into the blood, an injection would work just as well. If instead it delivers oxytocin to brain regions that peripheral circulation cannot easily reach, the nasal route is doing something truly unique.

Insulin presents a different angle. The brain uses insulin for signaling functions unrelated to blood sugar, including memory consolidation. In a clinical trial of adults with mild cognitive impairment or early-stage Alzheimer’s disease, daily intranasal treatment with 40 IU of insulin detemir improved memory composite scores compared to placebo. The response depended on a person’s genetic profile: carriers of the APOE-ε4 allele, the strongest genetic risk factor for Alzheimer’s, improved significantly, while non-carriers actually worsened.¹¹ In a rat model, six weeks of intranasal insulin prevented spatial memory impairment and reduced hallmarks of Alzheimer’s pathology, including tau hyperphosphorylation and microglial activation.¹² The idea that a whiff of insulin could slow cognitive decline is compelling, but the clinical data so far comes from small trials with genetically stratified responses, so it is far from settled.

Intranasal Vaccines and Mucosal Immunity

Most vaccines are injected into muscle, which produces antibodies in the bloodstream but relatively little immune protection at the mucosal surfaces where respiratory pathogens actually enter the body. The nose contains its own immune tissue, including structures collectively known as nasopharynx-associated lymphoid tissue, which in humans forms Waldeyer’s ring (the adenoids and tonsils). This tissue is specialized for generating B-cell responses and can drive production of both mucosal and systemic antibodies.¹³

A nasal vaccine against a respiratory pathogen like influenza mimics the natural route of infection and triggers production of immunoglobulin A (IgA) directly in the nasal cavity, creating a first line of defense at the point of viral entry. This mucosal IgA can neutralize a virus before it establishes infection, something systemic antibodies from an injected vaccine are less efficient at doing. However, nasal vaccines typically need adjuvants, substances that boost the immune response, because the nose’s mucociliary clearance tends to flush antigens away before the immune system fully responds.¹⁴ Finding adjuvants that are potent enough to work but gentle enough for the sensitive nasal lining has been one of the persistent challenges in this field.

Why Particle Size, Spray Angle, and Head Position Matter

Getting a drug into the nose is easy. Getting it to the right part of the nose is hard. The olfactory region, where direct brain access occurs, sits high up in the nasal vault, well above where a standard squeeze-bottle spray deposits most of its payload. Computational modeling of nasal spray deposition shows that when droplet size exceeds about 50 micrometers, roughly 80 percent of particles land in the front of the nose and never reach the deeper regions. Shrinking droplets to around 10 micrometers shifts deposition toward the middle and posterior regions, where absorption is more useful.¹⁵ A middle spray direction (neither angled too high nor too low) also improves delivery to the main nasal passage for particles in that 10-micrometer range.¹⁶

Device design plays a role as well. A bidirectional powder device, which uses the patient’s own exhalation to propel powder through one nostril and out the other, deposited substantially more drug in the upper and middle posterior regions of the nose compared to a traditional liquid spray pump. The powder device achieved about 53 percent deposition in those upper target zones, versus roughly 16 percent for the liquid spray. The liquid spray, by contrast, deposited nearly 60 percent of its dose in the lower anterior region, where absorption into the brain is least likely.¹⁷

Individual anatomy matters too. Minor septal deviations, narrow nasal valve areas, and swollen inferior turbinates all reduce deposition in the key middle-turbinate region where many topical steroids need to land. Tilting the head to the side can help bypass a minor septal deviation.¹⁸ The natural nasal cycle, a roughly alternating pattern of congestion and decongestion between the two nostrils, also influences where spray ends up, and tipping the head backward increases variability in deposition because gravity and liquid film instability start to play a bigger role.¹⁹

Does Nasal Congestion Block Drug Absorption?

A reasonable assumption is that a stuffy nose would sabotage intranasal drug delivery. If swollen tissue blocks airflow, wouldn’t it block drug absorption too? A systematic review examined this question specifically for intranasal rescue medications, the fast-acting drugs where reliable absorption is most critical. The evidence was reassuring: across six studies evaluating intranasal epinephrine, glucagon, fentanyl, and zolmitriptan, nasal congestion did not reduce absorption in any of them. Two of the epinephrine studies actually found that allergen-induced congestion increased peak blood levels, possibly because inflammation increases blood flow and vascular permeability in the swollen tissue. The authors concluded that congestion’s impact likely depends on each drug’s specific properties and formulation, but there is no blanket negative effect.²⁰

Safety Concerns for the Nasal Lining

Anything sprayed repeatedly into the nose has the potential to damage the delicate mucosal tissue and impair the cilia that keep the airway clean. A review of the literature on intranasal drugs found that some formulations do hamper ciliary function and mucociliary clearance, making safety testing for these effects essential before recommending chronic use.²¹ The concern is not limited to the active drug itself. Excipients (the inactive ingredients in a formulation) can be problematic. In one study, the mucoadhesive polymer Carbopol 971P caused dose-dependent slowing of ciliary beat frequency and, when applied to rabbit nasal tissue, produced severe inflammation that worsened with longer treatment, even though it did not cause outright tissue death or structural degeneration of the cilia.²²

In vitro testing using 3D nasal tissue models has shown that even a common corticosteroid spray like mometasone, applied daily at clinically relevant doses for two weeks, produces dose-dependent and cumulative reductions in ciliary activity along with ultrastructural signs of cell damage.²³ This does not necessarily mean that every patient using a steroid nasal spray is harming their nose; the in vitro setup exposes cells more directly than a living nose where mucus and blood flow provide some buffering. But it underscores why long-term nasal drug products need rigorous ciliotoxicity testing that simulates repeated exposure, not just single-dose safety data.

Lipid Nanoparticles and Formulation Engineering

One of the most active areas in intranasal research is the design of carrier systems that protect drugs from enzymatic breakdown, stick to the nasal mucosa long enough to be absorbed, and improve transport across the epithelium. Lipid-based nanoparticles, including nanoemulsions, liposomes, solid lipid nanoparticles, and nanostructured lipid carriers, have shown particular promise for nose-to-brain delivery.²⁴ These carriers work through several mechanisms at once: their lipid composition improves adhesion to the oily nasal mucosa, the particle shell shields the encapsulated drug from enzymatic attack, and some formulations can even evade transport proteins that would otherwise pump drugs back out of the cells.²⁵

Stimuli-responsive gels represent another frontier. These are liquid when sprayed but thicken into a gel upon contact with the warm, moist nasal environment, prolonging residence time without the patient feeling like they have a foreign substance lodged in their nose. Combining such gels with nanoparticle carriers is an area of especially active research, though most of this work remains preclinical.

Gene Therapy Through the Nose

Some of the most striking recent work involves using the nasal route to deliver gene therapy vectors to the brain. In a mouse study comparing intranasal to intravenous delivery of an adeno-associated virus (AAV9) vector, the nasal route achieved roughly 15 percent of the brain transduction efficiency seen with intravenous injection. That sounds modest until you consider the tradeoff: intranasal delivery reduced exposure to peripheral organs like the liver, stomach, kidney, and spleen by up to 13,400-fold compared to the intravenous route. When researchers calculated the ratio of brain transduction to peripheral organ transduction, the intranasal route actually came out ahead for several key organs, meaning a higher proportion of the delivered vector ended up where it was wanted (the brain) rather than where it could cause side effects.²⁶

A separate group combined intranasal AAV delivery with focused ultrasound to create a hybrid technique. The ultrasound temporarily loosened the tissue barriers, boosting brain delivery by roughly 2,000-fold compared to intranasal delivery alone and over 400-fold compared to the standard ultrasound-assisted blood-brain barrier opening used with intravenous injection.²⁷ All of this remains in animal models, but it hints at a future where gene therapies for neurological diseases might be delivered without surgery or systemic exposure.

The Nose as a Pathogen Highway

The same olfactory nerve anatomy that makes nose-to-brain drug delivery possible also provides a highway for pathogens. Each olfactory receptor neuron stretches a dendrite into the nasal cavity on one end and sends an axon through the cribriform plate into the brain on the other. A long list of viruses can exploit this shortcut, including influenza A, herpesviruses, poliovirus, rabies virus, West Nile virus, Japanese encephalitis virus, and several others.²⁸

This dual nature of the olfactory route creates a tension in intranasal drug development. Formulations designed to enhance transport across the olfactory epithelium could, in principle, also make it easier for pathogens to tag along. Conversely, understanding how viruses travel from nose to brain has informed how researchers design nanoparticle carriers and mucosal adjuvants: the goal is to borrow the transport mechanism without opening the door wider to infection. There is no evidence that using a standard intranasal medication increases your susceptibility to viral neuroinvasion, but the theoretical concern shapes how new formulations are safety-tested, particularly those designed to linger in the olfactory region.

Why So Few Nose-to-Brain Products Have Reached the Market

Given the promising science, it is fair to wonder why your pharmacy shelf is not overflowing with nose-to-brain therapies. The approved intranasal products that exist, like naloxone and esketamine, mostly rely on systemic absorption (naloxone) or are small molecules that cross membranes readily (esketamine). True nose-to-brain delivery of larger molecules remains largely a preclinical phenomenon. Reviews of the translational landscape consistently note a critical gap between experimental research and clinical reality: formulation development, device selection, and the lack of standardized methods for measuring how much drug actually reaches the brain in humans all slow the path to approval.²⁹

The variability of human nasal anatomy compounds the problem. Septal deviations, polyps, chronic inflammation, and even the hour-to-hour fluctuations of the nasal cycle mean that two patients using the same spray can get very different doses to the target tissue. Device manufacturers have historically underestimated this complexity. And in clinical management, conditions affecting the nose tend to be treated as secondary concerns, meaning olfactory dysfunction or anatomical abnormalities that could affect drug transport often go unaddressed. Until researchers develop better tools for predicting and measuring nasal drug deposition in individual patients, the field will continue to produce impressive animal data that stalls on the way to the clinic.