Why Nostrils Flare When You Smile, and How Botox Weakens the Muscle Behind It

Smile broadly in front of a mirror and some people notice the sides of the nose spreading outward, making the nostrils look bigger. It usually is not visible at rest and only shows up when smiling, talking, or breathing in deeply. This is called nostril flaring, and it has less to do with the shape of the nose itself than with how hard a small muscle beside the nose is pulling.

That is why the approach is different from procedures that reshape or narrow the nose. Nostril Botox does not touch bone or cartilage at all. It simply reduces the strength of the muscle that pulls the nostrils outward. It might seem strange that adjusting a single muscle can change how an expression looks, so let's go through what that muscle is and what Botox actually does to it.

Why Do Nostrils Flare When You Smile?

At rest, the nostrils barely move. The real change happens when you make an expression. When you smile broadly or talk, the muscles across the whole face move together, and the muscle beside the nose contracts along with them, pulling the nostrils outward and downward.

Something similar happens when you take a deep breath. When the body suddenly needs more air, a reflex widens the nostrils to let more air in. This is naturally meant to help with breathing, but in people where this muscle is especially developed or gets used often, the nostrils flare noticeably every time they make an expression.

In other words, nostril flaring is not a structural issue with the nose, it comes from an exaggerated muscle response. Compare a photo taken at rest with one taken while smiling and the shape of the nose is identical, yet the nostrils only look bigger in the smiling photo. That is exactly why.

The Muscle Responsible for Nostril Flaring

The movement comes from one of the nasalis muscles beside the nose, specifically the part called the dilator naris, named for its job of widening the nostrils. It sits as a thin sheet running along the rim of each nostril.

Picturing it as a rubber band makes it easier to understand. It stays loose most of the time, but the moment force is applied, it pulls taut and spreads the nostril to the side. Since the direction this muscle contracts lines up exactly with the direction that pulls the nostril outward, the harder it works, the wider the nostril flares.

Right beside it sits another muscle that lifts the upper lip while also tugging on the nostril. Because it pulls up both the upper lip and the side of the nostril at the same time, a broad smile often brings the corner of the mouth and the flare of the nostril in at the exact same moment. Since these two muscles pull from different spots but in a similar direction, how strongly each one acts varies from person to person, which is why the degree and shape of nostril flaring looks slightly different from one face to the next.

How Does Botox Reduce the Muscle's Strength?

Botox, or more precisely botulinum toxin, is not a drug that destroys the muscle. It works at the junction between the nerve and the muscle, blocking the nerve's ability to deliver the signal that tells the muscle to move.

Think of the muscle as a speaker and the nerve as the wire sending it a signal. Botox does not break the speaker, it weakens the signal traveling through the wire. Once the signal is weaker, the speaker, meaning the muscle, cannot respond as forcefully as before even when it does receive the command.

Nostril Botox applies this exact principle to the dilator naris. Injecting a small amount of Botox into the muscle weakens the nerve signal reaching it, and along with that, the force of the muscle's contraction when smiling or breathing deeply drops as well. The muscle is not completely disabled, it simply cannot generate enough force to yank the nostril outward. The result is normal breathing at rest, with only the flaring during a smile toned down.

When Does It Take Effect, and How Long Does It Last?

Blocking the signal between nerve and muscle takes time. The effect does not appear immediately after injection. Reports commonly describe the muscle's response gradually easing over a few days to one or two weeks, since the body needs time to reset how it transmits signals in that area.

Even once the effect settles in, it does not last forever. Over time, the body regrows new connections around the nerve endings that were blocked, and through that process the muscle gradually regains its original strength. Like Botox anywhere else on the face, nostril Botox is generally understood to hold its effect for a few months before gradually fading.

So this is not a one-time procedure, it needs to be repeated once the effect starts to wear off to keep the result going. Some reports suggest that with repeated sessions the muscle itself gradually thins, extending the interval between touch-ups a little each time, though this varies with each person's muscle thickness and habits.

Is There a Particular Case Where Nostril Botox Works Best?

Nostril flaring can stem from the muscle, from a naturally wide cartilage or bone structure, or from a mix of both. Since Botox only reduces muscle movement, it tends to show a clear result when the nose looks fine at rest but flares noticeably during a smile or other expressions.

On the other hand, if the nostrils already look wide at rest and stay that way regardless of expression, structure is likely playing a bigger role than muscle. In that case, reducing muscle strength alone may bring limited change. That is why it helps to compare the shape of the nostrils at rest and while smiling before deciding on the procedure.

It is also worth noting that people who have trouble breathing through the nose, or whose nose is frequently blocked due to conditions like rhinitis, may already be relying heavily on the dilator naris to breathe. Weakening this muscle could make breathing feel more uncomfortable, so checking breathing status beforehand is important.

What Else Gets Considered During the Procedure?

Since nostril Botox targets a small area where controlling the dose matters a great deal, the following points are commonly reviewed before treatment.

• Dose control: Injecting more than necessary can press the nostril down too far and make nasal breathing uncomfortable, so starting with a small amount and watching the response is the safer approach. The goal is to reduce the muscle's strength, not eliminate it entirely.
• Balance with neighboring muscles: If the muscle that lifts the upper lip is affected as well, the upper lip may not rise as much while smiling, so precise placement matters. Because the muscles sit close together, even a slightly off injection point can end up affecting the wrong one.
• Checking for left-right asymmetry: Many people flare more on one side than the other, so rather than injecting the same amount on both sides, the dose is often adjusted to match how each side naturally flares. This reflects the fact that the pulling force of the muscle has developed differently on each side to begin with.
• Checking breathing beforehand: If nasal congestion is frequent or the septum is deviated, the dilator naris may already be helping with breathing, so it is recommended to check this in advance. This helps gauge the chance that breathing could feel more uncomfortable once the muscle's strength is reduced.

How Should You Care for the Area Afterward?

For the first few days while Botox is settling in, it is recommended not to press or massage the injected area firmly. If the drug spreads before it has settled, it could weaken muscles beyond the intended area.

Some swelling or light bruising can appear in the first few days after the procedure, and it is generally described as settling down naturally over time. Because the skin over the nostril is thin and blood vessels run close to the surface, bruising here can be slightly more visible than in other areas.

During the one to two weeks it takes for the effect to appear, checking your smile in the mirror from time to time can help you track the change. Since each muscle responds at its own pace, it is often worth waiting a bit longer if the effect still feels insufficient during this window. And if breathing starts to feel uncomfortable, letting the clinic know right away is the safer move.

References

Korean Dermatological Association, American Academy of Dermatology (AAD), and Ministry of Food and Drug Safety (MFDS).