Patients ask this more than almost any other question about cosmetic injectables. The mechanism is genuinely interesting once you understand it, and most of the marketing language gets it slightly wrong.
If you’ve been reading up on how anti-wrinkle injections work, you’ve probably come across phrases like “freezing the muscle” or “paralysing the nerves”. Neither is quite accurate. The actual mechanism is more elegant and more localised than the metaphors suggest.
The signal between nerve and muscle
Every time you frown, smile, or raise your eyebrows, your brain sends an electrical signal down a motor nerve to the relevant muscle. That signal can’t cross the tiny gap between the nerve ending and the muscle on its own. It needs a chemical messenger, called acetylcholine, to carry it across that gap (the neuromuscular junction, in clinical terms).
Acetylcholine is released from the nerve, drifts across the gap, binds to receptors on the muscle, and triggers contraction. The whole process takes milliseconds and happens millions of times a day across every muscle in your body without you ever thinking about it.
Anti-wrinkle injections interrupt that messenger system. Specifically, they prevent acetylcholine from being released from the nerve ending in the first place.
In our experience, patients find this part of the explanation oddly reassuring. The mechanism is precise and reversible. Nothing is destroyed, nothing is permanently altered, and the body’s own biology eventually restores normal function.
What the active ingredient does
The active ingredient is a purified neurotoxin produced by a specific bacterium. (We’re not going to use the brand or active term here for regulatory reasons. The General Medical Council and the Advertising Standards Authority limit what UK clinics can say in consumer-facing material about prescription-only medicines.)
What it does, biochemically, is bind to a protein called SNAP-25 inside the nerve terminal. SNAP-25 is part of the machinery that releases acetylcholine into the synaptic gap. Once it’s blocked, the nerve simply can’t deliver its messenger to the muscle. The signal is generated, sent, and then quietly absorbed without producing a contraction.
The Wikipedia entry on the neuromuscular junction covers the underlying biology in more detail if you want the full mechanism. That’s the mechanism. The muscle isn’t damaged. The nerve isn’t damaged. The signal pathway is just temporarily interrupted.
Why “freezing” is the wrong word
The treated muscle isn’t frozen. It’s quiet. There’s a real difference, and it matters for setting expectations. A frozen muscle would be locked in one position. A quiet muscle simply doesn’t activate when asked. You can still feel the area normally. You can still move surrounding muscles. The relaxation is selective and it builds up gradually as more nerve endings get blocked.
Actually, this is why the effect takes days to appear rather than minutes. The blocking happens at a microscopic level across countless individual nerve terminals. It takes time for enough of them to be silenced before the visible effect emerges.
Why the lines fade
Wrinkles in the upper face are caused by repeated muscle contraction over years. Every time you frown, the skin above the active muscle creases. With enough repetition (decades, in most cases), those creases set into permanent lines.
When the muscle stops contracting, the skin above it stops being repeatedly folded. Over the weeks following treatment, the established lines soften because they’re no longer being reinforced. The deeper the line, the slower the softening, but most patients see meaningful improvement within two to three weeks of treatment.
Lines that have set very deeply (the kind you can see on a relaxed face, not just an animated one) often soften but don’t fully disappear. We’d usually flag that during consultation.
This is one of the biggest sources of misaligned expectation. Patients see before-and-after photos online and assume the deepest lines will vanish entirely. They usually soften meaningfully, but the skin itself has been creased for years and won’t always rebound completely.
Why the effect wears off
This is where the biology gets quietly impressive. The body doesn’t accept blocked nerve terminals as a permanent state. Instead, the affected nerves grow new terminal sprouts that bypass the blocked machinery and start releasing acetylcholine again.
It’s a slow process. It takes somewhere between three and four months for enough new nerve endings to take over that muscle function returns to baseline. Some patients regain function slightly faster, others slightly slower. Metabolism, muscle bulk, and dose all play a role.
We had a patient who genuinely thought her treatment hadn’t worn off because she’d stopped frowning out of habit during the months of relaxed muscle activity. When the muscle came back online, she had to consciously retrain herself to keep her brow softer. That’s actually one of the more interesting secondary benefits patients report.
How dose translates to effect
The product is measured in units. Each unit corresponds to a defined amount of active ingredient. Different muscles need different doses depending on their bulk and the strength of their habitual contraction.
| Muscle group | Function | Approximate dose range | Typical onset |
|---|---|---|---|
| Frontalis (forehead) | Raises eyebrows | 10 to 20 units | 5 to 10 days |
| Corrugator (frown) | Pulls brows together | 15 to 25 units | 3 to 7 days |
| Orbicularis oculi (eye) | Closes the eyelid, creates crow’s feet | 10 to 15 units per side | 3 to 7 days |
| Masseter (jaw) | Closes the jaw, used in clenching | 20 to 40 units per side | 2 to 4 weeks |
Dose isn’t a precise science. Two patients with apparently identical anatomy can respond differently to the same dose. In practice, we tend to start conservatively on a first treatment and adjust upwards on subsequent visits if the effect is too subtle. Going too strong on a first attempt is harder to undo than going too gentle.
It’s also worth knowing that not every part of a muscle needs to be reached. Most facial muscles are broad sheets, and the visible line corresponds to a specific zone of contraction. A skilled injector targets that zone rather than blanketing the whole muscle. The result is a softening of the line without an unnatural flattening of the surrounding tissue. Patients who’ve had this treatment delivered well often say they look rested rather than treated. Patients who’ve had it delivered poorly tend to look slightly stiff in animation, which is usually a sign that too many fibres have been quieted at once.
Patients often ask why their first round seemed to last longer (or shorter) than the second. Several factors feed into this. Sleep quality, stress levels, intense exercise routines, and the season of the year all show up in our records as variables that correlate with treatment longevity. Heavy training in the gym, in particular, seems to shorten the effect for some patients, possibly because elevated metabolism processes the active ingredient more quickly. Stress correlates the other way: patients going through busy work periods often clench more, which can sometimes shorten visible effect even though the underlying mechanism hasn’t changed.
None of this is something to worry about. It’s just a reminder that the body is doing its own thing and the treatment is one input among many.
What the treatment can’t do
Anti-wrinkle injections relax muscle activity. They don’t fill volume, they don’t tighten skin, and they don’t address sun damage or pigmentation. Lines caused by ageing of the skin itself rather than muscle activity won’t respond to this treatment.
Some patients find this completely straightforward. Others come in expecting a result that the product simply can’t deliver. The conversation about what’s achievable is worth having before any needle goes near the skin.
You’ll find articles online claiming the treatment works on every type of facial line. In our experience, that’s not the case. Static lines (visible at rest, with the muscle relaxed) respond poorly. Dynamic lines (visible during expression) respond well. The distinction matters and any thorough consultation will identify which type of line you’re actually dealing with.
Is the mechanism the only thing that matters? Not really. Technique matters. Dose matters. Anatomical knowledge matters. Two providers using the same product can deliver very different results.
If you’re curious about whether this would suit your face, our team at Westcliff Dental Practice is happy to talk it through in detail. There’s no obligation to proceed and we’ll give you an honest answer about what we’d expect. Call us on 01702 344 500 to book a consultation.
