Filler side effects and botulinum toxin complications: what to know

Filler and toxin side effects: the most common

A meta-analysis published in 2026, pooling 42 studies, gives an order of magnitude. A treatment-related side effect was reported in about six people in ten after hyaluronic acid filler (lips, folds, lower face), with wide differences between studies, and in nearly one person in five after botulinum toxin in the upper face. They were mostly mild to moderate and short-lived:

• after hyaluronic acid: swelling, pain, bruising, redness, tenderness;
• after botulinum toxin: mainly headache, sometimes pain or a bruise at the injection site.

Generally, these effects appear early and go away on their own. It is simply wiser not to schedule a treatment just before an important event.

After toxin: a drooping eyelid or eyebrow

The most feared side effect of the toxin is a drooping eyelid or eyebrow, when the product spreads to a neighbouring muscle. A review of 35 studies, covering nearly 8,800 people, put figures on it for the upper face: about 2.5% for the eyelid and 3% for the eyebrow. All these effects resolved on their own.

Very rarely, the toxin can act at a distance from the injection site: difficulty swallowing, speaking or breathing, unusual muscle weakness. These effects are very rare, but they need immediate medical advice.

After hyaluronic acid: vascular occlusion

The serious complication of hyaluronic acid is vascular occlusion: the gel blocks a blood vessel, and the skin it supplies is starved of blood. If not treated quickly, it can damage the skin. It is rare: in a survey of 370 US dermatologists covering 1.7 million syringes, it occurred about once per 6,400 syringes injected with a needle and once per 41,000 with a cannula, and 85% of cases left no lasting after-effects.

Exceptionally, the gel can reach the artery of the eye and cause loss of vision, sometimes irreversible. An international consensus estimates this risk at less than one case per 100,000 syringes. The areas most often involved in published cases, all fillers combined, are the nose, the area between the eyebrows, the forehead and the nasolabial folds.

How I prevent them

• Knowing the anatomy: each area has its own vessels and its own safe injection depth. The forehead, the nose and the area between the eyebrows are classed as very high risk of visual complications for hyaluronic acid.
• Injecting very slowly, at low pressure, in small amounts (less than 0.1 ml at a time), keeping the needle moving.
• Choosing between cannula and needle according to the area: the blunt-tipped cannula is associated with fewer occlusions, but it is not safer everywhere, for example on the nose.
• Not relying on the aspiration test, which consists of pulling back the plunger before injecting: this step is debated, and an international consensus considers it unreliable.
• Always having hyaluronidase, the enzyme that dissolves the gel, at the practice, as experts recommend, along with a written emergency protocol.

And, the first time, small amounts: we can complete at the check-up around day 15. I explain in another article what hyaluronidase can and cannot do.

Signs that mean you should call immediately

After a hyaluronic acid injection, call the practice immediately on +32 492 82 82 62, or 112 if you cannot reach us, if you notice:

• intense or unusual pain;
• skin that turns pale, mottled or purplish in or around the treated area;
• a visual disturbance, double vision or a drooping eyelid just after the treatment: also call 112.

After botulinum toxin, difficulty swallowing, speaking or breathing means calling 112. The same applies, after any treatment, to weakness on one side of the body or sudden difficulty speaking.