Asymmetric smile after Botox: why, how long and what to do

One corner of the mouth lifting less than the other, an upper lip that stays put on one side when you smile, a mouth that looks "crooked" in photos: after a botulinum toxin injection, the smile can lose its symmetry. This is one of the smile problems after an injection. It is visible and can be bothersome day to day, but it is temporary. Here are the answers to the seven questions people ask most often.

Why does the smile become asymmetric?

Smiling uses several muscles on each side of the face: the zygomaticus major and minor pull the corner of the mouth upwards, the elevator muscles lift the upper lip, and other muscles lower the corner of the mouth and the lower lip. When the toxin reaches one of these muscles on one side only, or acts more strongly on one side than on the other, the two halves of the smile no longer move in the same way.

The sources describe several origins:

• after an injection for crow’s feet: the label of a type A toxin recommends avoiding injections too close to the zygomaticus major muscle to prevent the lip from drooping; it lists facial asymmetry among uncommon effects¹;
• after an injection at the corner of the mouth: according to an expert consensus on the lower face, an asymmetric smile is the most frequent complication of treating the muscle that pulls down the corner of the mouth, through spread to the muscle that lowers the lower lip³;
• after treatment of a gummy smile: an asymmetric smile is among the complications described⁴.

In a review of 35 clinical studies, lip asymmetry and imbalance were the side effect most often reported after lower-face treatments². The US label of a type A toxin also points out that muscles next to the targeted ones can be weakened by spread⁵.

How soon after the injection does it appear?

It appears as the effect of the toxin builds up. Side effects are usually seen in the first few days after the injection⁶, and within the first week according to the US label⁵. No source gives a timeframe specific to the smile.

How long does it last?

It lasts at most as long as the effect of the toxin in the muscle affected. In the review of 35 studies, all lip asymmetries resolved on their own². For crow’s feet, the effect of a type A toxin has been shown for a median of 4 months⁶. The details are explained on the page about how long botulinum toxin lasts.

Is it reversible?

Yes. The asymmetry disappears when the affected muscle regains its strength. After an injection, transmission between nerve and muscle is gradually restored, first through small nerve sprouts, then by the return of the original nerve ending⁸.

Is there a treatment?

In most cases, you wait for recovery. When the asymmetry is marked, a correction may be discussed: an expert consensus describes a small injection into the matching muscle on the opposite side, to rebalance the smile³, and a case report describes an asymmetry of the corner of the mouth corrected this way in two weeks⁷. This level of evidence is low, with no comparative trial. The correction does not "repair" the affected side: it weakens the other side to even things out, which is not suitable in every situation. No massage, cream or exercise has shown any effect in the sources we consulted.

What should you avoid?

• Do not ask for a correction before the effect of the toxin has fully built up: the asymmetry can still change over the first few weeks.
• Do not have injections one after another: the effect of different toxins given a few months apart is not known, and weakness can be made worse by another toxin injected before the effect of the previous one has worn off⁵.
• Do not force grimaces or exercises to "wake up" the muscle: no source shows that they speed up recovery.

When should you see a doctor?

Talk to the doctor who performed the injection as soon as the asymmetry bothers you, and sooner if you have trouble eating, drinking, closing your mouth or speaking. Write down the date of the injection, the areas treated and the day the asymmetry appeared; dated photos at rest and smiling will help track how it evolves. Difficulty swallowing, a changed voice, difficulty breathing or a sudden paralysis of one whole side of the face are warning signs: call emergency services (15 or 112 in France, or your local emergency number).

If your smile looks mainly less natural, without being crooked, see the page on the smile that has changed after an injection. If the asymmetry mainly affects the lips at rest, the page on uneven lips after an injection is more relevant.