Rumour: “Botulinum toxin (commonly called ‘botox’) wastes the muscles in the long term and makes the face sunken” — What does the science say?

Short answer: partly true, but the conclusion is exaggerated. A muscle treated with botulinum toxin can become somewhat thinner because it works less; in the jaw muscle this is an effect that is deliberately aimed for. Reviews of the subject describe this thinning as temporary and reversible. On the other hand, the long-term effect on the muscles of facial expression of treatments repeated for years has not been adequately researched. A face looking 'sunken' over the years, meanwhile, is usually due not to muscle thinning but to the changes in bone and fat seen with age, and to weight loss.

Where does the rumour come from?

The observations on which the rumour rests are real. First, everyone knows that a muscle that is not used becomes thinner; most of us have seen an arm that has been in plaster grow thin. Second, it is known that botulinum toxin is used to slim the jaw muscle and that this gives a visible result. Third, the faces of some people who have been having treatment for years may look thinner and more hollow over time; the cause of this change is readily attributed to botulinum toxin.

The first two observations are correct. With the third, it is overlooked that age has also advanced over the same years.

Does botulinum toxin really thin the muscle?

Yes. Botulinum toxin temporarily reduces the contraction signal passing from the nerve to the muscle; a muscle that works less loses volume over time. The American Academy of Dermatology also states that with repeated treatments for lines the muscle may become thinner and that this may lead to the effect lasting longer. In other words, muscle thinning is not a concealed side effect but a natural extension of the way the medicine works. For the general properties of the medicine, see our article “Botulinum Toxin”.

A plastic surgery review of the subject states that muscle thinning occurs after botulinum toxin, that in many publications it is reported as temporary and reversible, and that physicians need to be prepared to inform their patients about it. The same review also describes examples of unwanted thinning: hollowing seen in the temple area after treatments given for migraine, and thinning of several facial muscles in a patient treated for hemifacial spasm. These examples show that thinning may not occur only in the targeted place and only to the desired extent.

Thinning of the jaw muscle: an effect that is deliberately aimed for

In treatments of the chewing muscle (the masseter), thinning of the muscle is often the aim itself. A 2026 meta-analysis covering five randomised, placebo-controlled studies and 536 participants reported that botulinum toxin reduced muscle thickness compared with placebo and improved prominence at the angle of the jaw in the short term. The authors also state that the differences between the studies were large and that studies with longer follow-up are needed.

On recovery in the chewing muscles, the data are more cautious. A systematic review of tissue changes in the jaw muscle reported thinning of the muscle and changes in fibre structure, and stated that recovery was variable and at times incomplete. However, only two of the fourteen studies in this review were carried out in humans; the authors therefore emphasise that the results need to be interpreted with care and cannot be directly generalised to patients.

When the chewing muscle weakens, the load on the lower jawbone also decreases. The data on the possible effect of this on the bone are disputed, and its clinical significance is unknown; we deal with this subject and other details specific to the jaw muscle in our article “Masseter (Jaw Muscle) Botulinum Toxin”.

What do long-term data show for the muscles of facial expression?

The muscles of facial expression in the forehead, between the eyebrows and around the eyes are much thinner and smaller than the chewing muscles; measuring changes in volume in these muscles is difficult, and there are few studies on the subject.

In one of them, two men who had never had treatment before were given a single treatment between the eyebrows, and the muscles were followed for a year with magnetic resonance imaging. A reduction of about 46 to 48 per cent was found in the volume of a small muscle between the eyebrows, and this reduction was still present after twelve months; the lines, meanwhile, returned to their pre-treatment state within six to ten months. In other words, the volume of the muscle stayed low for longer than the visible effect. This finding is interesting; however, it rests on only two people and is not enough to generalise. The authors also state that this difference between the muscle's volume and its function needs to be explained.

A neurology review on muscle thinning due to botulinum toxin, for its part, concluded after searching more than five hundred publications that thinning developing after long-term treatment is documented surprisingly little in the literature, and that the mechanisms and the clinical factors leading to thinning have not been adequately studied. This review focuses predominantly on neurological diseases, that is, on treatments given to different muscles from cosmetic treatments and mostly in larger amounts.

In summary: it is known that thinning occurs in the muscles of facial expression and that muscle movement returns when treatment is stopped. Whether the volume of the muscle returns completely to its former state after cosmetic treatments repeated regularly over decades has not been shown by well-designed studies. The honest answer is 'most probably it largely returns, but we do not know for certain'.

Why does the face look 'sunken'?

The main structures that determine the fullness of the face are not the muscles of facial expression. These muscles lie in thin layers; the volume of the face is made up chiefly of the bony skeleton and the fat tissue over it. The perception of hollowing seen over the years usually has other explanations.

We have dealt with the same comparison error for fillers too: “Rumour: If you stop fillers, your face will sag more than before — What does the science say?”. For age-related volume loss and the causes of sagging, see our article “Sagging of the Face and Neck”.

That said, it would not be right to say that muscle thinning is never reflected in appearance. In areas where the muscle contributes to the outer contour, such as the temple and the angle of the jaw, thinning can be noticed by eye. In the jaw muscle this is usually the desired result; at the temple, or in a jawline that has thinned more than necessary, it may be perceived as unwanted hollowing. Changes in facial expression are a separate matter and are explained in our article “Rumour: Botulinum toxin (commonly called botox) makes your face look frozen — What does the science say?”.

What happens when you stop botulinum toxin?

When treatment is stopped, the nerve endings re-establish their connections over time and muscle movement returns within months. A muscle that starts working again is also expected to regain its volume over time; this is the basis on which publications describe the thinning as reversible. The lines return to their pre-treatment state and to the natural course of ageing; there is no evidence showing that the face becomes worse than before because treatment was stopped.

No special procedure or 'tapering off' is needed in order to stop; it is enough not to have a new treatment. We have also dealt with this subject in our article “Rumour: Starting botulinum toxin (commonly called botox) at a young age prevents wrinkles — What does the science say?”.

The unknowns of very frequent and large treatments

• Long-term data are limited: there are no large studies following the effect on muscle structure of regular cosmetic use lasting decades.
• Recovery in the chewing muscles is uncertain: the systematic review mentioned above reports that the changes in the muscle do not always resolve completely over time; however, most of the data come from animal studies.
• Amount and frequency may matter: animal studies suggest that thinning depends on the amount; in humans this relationship has not been well defined.
• Neighbouring muscles may be affected: spread of the medicine to muscles other than the target can lead to unwanted weakness. The details are in our article “Side Effects after Botulinum Toxin”.

These uncertainties are not a definite reason to avoid treatment; but they are a good reason to avoid treatment that is more frequent or larger than necessary. Rather than asking for a new treatment while the effect is still present, increasing the amount each time or adding new areas, set the timing and the amount with your physician each time according to the findings on examination.

Conclusion

Botulinum toxin can thin the muscle into which it is injected; this is a known effect, and one that is used deliberately in the jaw muscle. The available publications mostly describe the thinning as temporary and reversible, but the long-term effect on the muscles of facial expression of cosmetic treatments repeated for years has been examined in few, small studies. A face looking sunken over the years usually has to do with the changes in bone and fat seen with age, and with weight loss. A moderate amount, intervals based on examination and taking a break when needed are the sensible approach in the face of the unknowns.