These complications are rare and primarily caused by temporary denervation of adjacent muscles outside of the intended treatment area. They can be technique-dependent; incidence declines as injector skill improves. Temporary blepharoptosis (upper eyelid droop) is uncommon (1% to 5%) but is distressing for patients. This is caused by the deep migration of toxins to the muscle responsible for raising the upper eyelid. This side effect of toxin can be treated using an ophthalmic solution. Eyebrow ptosis (droop) and undesired eyebrow shape are usually related to unintended toxin effects in the forehead muscle. Some of these complications can be corrected in a follow-up session; however, complications caused by the involvement of adjacent muscles are temporary and will spontaneously resolve as botulinum toxin effects diminish. Consistent technique and careful attention to injection volumes at the time of treatment and the use of ultrasound devices in the complicated areas can reduce the incidence of asymmetries. Other rare complications associated with toxin injections include the formation of antibodies (less than 1%), which can render treatments ineffective. Medications that inhibit neuromuscular signaling, such as aminoglycosides, quinidine, anticholinergics, and muscle relaxants, may potentiate toxin effects. Distant spread has not been reported with cosmetic use of botulinum toxin for frown lines or other facial indications.