Botox side effects: what is normal, what should worry you

After a botulinum toxin injection, it is common to notice a few reactions in the days that follow. Most are expected and harmless; some affect a muscle next to the treated area; very rare signs require urgent care. This page is part of our guide to problems after a botulinum toxin injection and is based on the summaries of product characteristics (SmPCs), the official labels written for doctors.

How the labels classify adverse effects

The labels classify each adverse effect by how often it occurred in trials: "very common" when it affects at least 1 patient in 10, "common" between 1 in 100 and 1 in 10, "uncommon" between 1 in 1,000 and 1 in 100, "rare" between 1 in 10,000 and 1 in 1,000^1,2. These frequencies depend on the product, the dose and the area treated. That is why figures for two products from different trials cannot be compared directly.

One type A toxin label states that adverse effects are seen during the first week after injection and are transient³. Another states that most were reversible and mild to moderate⁴.

Reactions at the injection site

These are the most common. The labels associate botulinum toxin injections with localised pain, inflammation, tenderness, swelling, redness, bleeding, bruising, warmth or induration, meaning a small firmer area under the skin¹. For frown lines, one label classes bruising, pain and swelling at the injection site as common²; around the eyes, bruising at the injection site is common^2,3.

The "lump" sometimes felt just after the injection is most often this local swelling. It settles on its own. A bruise follows its usual course, from purple to yellow, as after a blood test.

Pain or anxiety about the needle can also cause a brief faint, with a drop in blood pressure and sometimes a short loss of consciousness¹. This vasovagal reaction is not caused by the action of the toxin itself.

Headache

Headache is one of the most frequently reported adverse effects. For frown lines, one label classes it as very common⁴; others class it as common, for frown lines as well as forehead lines^2,3. The labels do not state how long it usually lasts. A sudden, very intense headache, or one combined with other signs such as visual disturbance, is not an expected effect: it should prompt you to see a doctor.

Effects related to the treated area

The toxin relaxes the injected muscles, but its effect can reach a neighbouring muscle. Depending on the area, the labels describe in particular:

• a drooping eyelid (eyelid ptosis), common in one type A label after treatment of frown lines and the forehead²;
• a drooping eyebrow or a feeling of heaviness in the forehead or eyelids^2,3;
• an eyebrow raised at the tail, called the Mephisto sign (lateral elevation of the eyebrow)^2,3;
• eyelid swelling and dry eye, common after treatment of crow’s feet with one product³;
• facial asymmetry or blurred vision, uncommon³.

For the short-acting botulinum toxin (serotype E), approved only for frown lines, the label reports a drooping eyelid in 0.17% of patients, a drooping eyebrow in 0.13% and a Mephisto sign in 0.04%¹. As of 9 October 2026, this toxin is approved in the European Union (since 15 July 2026) and in Canada (since 22 June 2026)^5,6. Its figures cannot be compared with those of type A toxins, which come from other trials.

In the vast majority of cases, these effects disappear as the effect of the toxin wears off. Each has its own detailed page in the atlas, for example for an eyelid that droops or swells after an injection.

More general effects

One label reports flu-like symptoms since the product was marketed, with no known frequency³. Nausea is described as uncommon after treatment of upper-face lines (forehead, frown lines, crow’s feet)³. A severe allergic reaction (anaphylaxis) can occur very rarely after a botulinum toxin injection¹: this is why the doctor keeps the necessary emergency equipment at hand.

Signs that should raise concern

Very rarely, the effect of the toxin is seen away from the injection site. The labels then describe muscle weakness, difficulty swallowing and speaking, and breathing difficulties^1,2. They state that difficulty breathing or swallowing is serious and can be fatal², and ask patients to seek immediate medical care if these signs appear^1,3. They can appear late: the symptoms of an overdose do not always show immediately after the injection¹.

Double vision is one of the signs to report without delay³. Warm redness that spreads, increasing pain or a fever may indicate an infection at the injection site¹.

Who is more at risk?

The labels contraindicate injection in case of known allergy to the product, a generalised disorder of muscle activity such as myasthenia gravis or Lambert-Eaton syndrome, or infection at the injection site^1,2. They advise against injection in people with a history of swallowing problems^1,2 and call for caution in case of neuromuscular disease, even if not yet diagnosed¹.

Anticoagulant treatment also calls for caution³: tell your doctor about it, but never stop it without the advice of the doctor who prescribed it. Some medicines, such as aminoglycoside antibiotics or muscle relaxants, can strengthen the effect of the toxin¹. Injection is not recommended during pregnancy^1,2. Hence the importance of mentioning your treatments, your medical history and any recent toxin injection before the procedure.

What to do if you have a side effect

For a reaction at the injection site or a moderate headache, waiting is usually enough. For an effect on a neighbouring muscle, get back in touch with the doctor who performed the injection: they know the area treated and the product used, and can judge what to do. If the expected effect does not happen, the question is different: see our page on botulinum toxin that does not seem to work. If, on the contrary, the effect seems too strong, read what to do when the effect of the toxin is too strong.

You can also report an adverse effect yourself. In France, the patient leaflet of one type A toxin explains that you can report it directly through the national reporting system of the ANSM (the French medicines agency) and the regional pharmacovigilance centres, as well as telling your doctor or pharmacist². If you were treated in another country, ask your doctor or pharmacist how to report it there.

If you have difficulty swallowing, speaking or breathing, call emergency services straight away (15 or 112 in France, or your local emergency number).