What are the possible adverse effects of botulinum toxin?

Updated: Mar 11, 2019
  • Author: Heather Rachel Davids, MD; Chief Editor: Elizabeth A Moberg-Wolff, MD  more...
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Since the mechanism of action of BoNT-A is so specific, adverse effects are uncommon and systemic effects rare. Flulike syndrome has been reported, but it is generally short-lived. Other adverse effects have been reported, but they are not necessarily a result of BoNT-A treatment. They include muscle soreness, headaches, light-headedness, fever, chills, hypertension, weakness, diarrhea, and abdominal pain.

Muscular weakness, the predominant and desirable effect of botulinum toxin injection, also may be considered an adverse effect when weakness occurs in an unintended area or is greater than intended. Clinicians must understand the functional consequences of unintended weakness caused by botulinum toxin injection. While overweakening the muscles that curl the toes may have few, if any, undesirable consequences, spread of toxin into the muscles that control swallowing, which can occur when injecting muscles near the larynx (for instance, the proximal part of the sternocleidomastoid muscle), may result in difficulty swallowing. Patients, therefore, should be informed of the potential for either too much weakness in the injected area or weakness in nearby muscles.

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