How does Botox work? The mechanism of botulinum toxin

Before talking about onset, duration or treatment areas, it helps to understand what the toxin actually does in the muscle. This mechanism is one of the basics covered in our overview of what botulinum toxin is. It explains almost everything else: why the effect is not immediate, why it stays local, and why it eventually wears off.

How does a muscle contract?

Every facial muscle is controlled by a nerve. When you frown, your brain sends an order that travels down the nerve to its tip, called the nerve ending, which sits against the muscle. This point of contact between nerve and muscle is called the neuromuscular junction.

At that point, the electrical order has to be converted into a chemical signal. The nerve ending holds tiny stores, called vesicles, filled with a messenger called acetylcholine. When the order arrives, these vesicles open onto the membrane of the nerve ending and release acetylcholine, which binds to the muscle and triggers its contraction^1,2.

For the vesicles to open, the nerve ending uses a small set of proteins called SNARE proteins. One of them, SNAP-25, is essential for the vesicles to dock and fuse with the membrane^1,2.

What does botulinum toxin do?

Botulinum toxin prevents the release of acetylcholine. Without this messenger, the nerve’s order no longer reaches the muscle, which contracts less^1,2. The label of the short-acting botulinum toxin (serotype E) describes the result as "partial chemical denervation and a localised reduction in muscle activity"².

The word "partial" matters. In aesthetic use, the aim is not to immobilise a muscle completely, but to reduce the strength of its contraction so that the line it creates softens. How strong the effect is depends on the dose and the injection points, chosen by the doctor.

The steps of the action, one by one

The labels and the scientific literature describe the toxin’s action as a series of successive steps^1,2,3. A reference review divides them into five stages: binding, entry into the nerve ending, passage of the active part into the cell, release of that active part, then cutting of the target protein³. They can be summarised as follows.

1. Binding to the nerve ending

After injection into the muscle, the toxin binds rapidly and very strongly to specific receptors on the surface of motor nerve endings^1,2. This binding is selective: the toxin recognises the endings of the nerves that control muscles³.

2. Entry into the nerve ending

The toxin is then carried inside the nerve ending, enclosed in a small pocket of membrane: this is endocytosis^1,2. To do so, it takes advantage of the natural recycling of vesicles that have just released their acetylcholine³.

3. Release of the active part

The toxin is made of two linked parts: a heavy chain, used to bind and enter, and a light chain, which is the active part³. Inside the nerve ending, the light chain crosses the wall of the pocket and ends up free inside the cell^2,3.

4. Cutting SNAP-25

The light chain behaves like molecular scissors: it cuts the SNAP-25 protein^1,2. Once SNAP-25 is damaged, the vesicles can no longer fuse normally with the membrane, and acetylcholine is no longer released^2,3. The muscle’s contraction decreases.

Why doesn’t the effect appear straight away?

All these steps take time. The label of a type A toxin explains that the last step is accompanied by a gradual inhibition of acetylcholine release¹. In practice, with a type A toxin, the median time to first effects is 2–3 days⁵. With the short-acting toxin, clinical effects begin to show 8 hours after the injection².

The effect then keeps building gradually until it peaks. The detailed figures, and the difference between first effects and peak effect, are explained on our page on the onset of action of botulinum toxin.

Why does the effect stay local?

The toxin acts where it is injected, on the nerve endings of the muscles it reaches^1,2. This is what makes it possible to target a specific muscle, for example the one that creates frown lines, without changing the others.

This precision has its limits, though. If the toxin reaches a neighbouring muscle, that muscle may relax too: this is, for example, how the drooping eyelid described in the labels happens, linked to injection technique¹. Much more rarely, effects away from the injection site have been reported; the labels ask patients to seek immediate medical care if they have difficulty swallowing, speaking or breathing^1,2. The volume injected and the dilution are among the factors that influence how far the effect spreads³.

Why is the effect temporary?

The toxin does not destroy the nerve. The blocked nerve ending responds by growing small branches, called sprouts, which reach out to the muscle and restore transmission^3,4. In a second phase, the original nerve ending recovers its ability to release acetylcholine, and the branches that are no longer needed disappear^3,4.

The label of a type A toxin places recovery within about 12 weeks, as the nerve endings restore their connections¹. The label of the short-acting toxin states that recovery generally occurs within 2–3 weeks, as primary nerve terminals recover². Why these durations differ so much, and what makes duration vary from one person to another, is detailed on our page on how long the effect of the toxin persists.

Long-acting and short-acting: same mechanism, two profiles

Type A toxins and the short-acting toxin both cut the SNAP-25 protein^2,3. Their overall mechanism is therefore the same. They differ in the exact place where they cut SNAP-25 and in how long their active part stays in the nerve ending: that of the type A toxin persists much longer, whereas that of the short-acting toxin is cleared faster by the cell’s breakdown systems³.

As of 9 October 2026, the short-acting toxin is approved in the European Union (since 15 July 2026) and in Canada (since 22 June 2026), for frown lines only^6,7. These molecular differences, and the other serotypes of botulinum toxin, are presented on our page on botulinum toxin serotypes and research.

What botulinum toxin does not do

Understanding the mechanism also tells you what not to expect from the toxin. It acts on muscle contraction, and therefore on lines that appear with movement. It does not fill a hollow, does not restore volume and does not treat sagging skin. A very deep line, visible even at rest, may only soften partly. These limits, like the choice of areas and doses, are to be assessed with your doctor during the consultation.