Side effects, which are generally minor and temporary, depend upon the mode of action (muscle paralysis) and chemical structure (protein) of the molecule, resulting in two major areas of side effects: paralysis of the wrong muscle group and allergic reaction.
Bruising at the site of injection is a side effect not of the toxin, but rather the mode of administration. In cosmetic use, this can result in inappropriate facial expression such as drooping eyelids, double vision, uneven smile, or loss of the ability to close eyes. This will wear off in around six weeks. Bruising is prevented by the clinician applying pressure to the injection site, but may still occur, and will last around 7–11 days.
When injecting the masseter muscle of the jaw, loss of muscle function will result in a loss or reduction of power to chew solid foods. All cosmetic treatments are of limited duration, and can be as short a period as six weeks, but usually the effective period lasts from two to three months. At the extremely low doses used medicinally, botulinum toxin has a very low degree of human and animal toxicity.
Other adverse events from cosmetic use include headaches, dysphagia, flu-like syndromes, blurred vision, dry mouth, fatigue, allergic reactions and swelling or redness at the injection site.