Hypersensitivity and anaphylactic reactions may occur rarely. Higher doses or more frequent administration may result in neutralizing antibody formation and loss of efficacy. Rarely, arrhythmia and myocardial infarction reported, sometimes in pre-existing cardiovascular disease. Weakness of adjacent muscles commonly reported. Dysphagia is common when used for cervical dystonia (risk factors: smaller neck muscle mass, bilateral injections into the sternocleidomastoid muscle, or injections into the levator scapulae). Excessive weakness of target muscles may also occur. Caution if there is excessive weakness or atrophy at the proposed injection site(s). Use is contraindicated if infection is present. Caution in patients with neuromuscular diseases and neuropathic disorders (such as amyotrophic lateral sclerosis), or those who are receiving other agents that may interfere with neuromuscular transmission (e.g. aminoglycosides, neuromuscular-blocking agents). Reduced blinking from injection of the orbicularis muscle can lead to corneal exposure and ulceration when treating blepharospasm. Retrobulbar hemorrhages may occur from needle penetration into orbit when treating strabismus; spatial disorientation, double vision, or past pointing may occur if one or more extraocular muscles are paralyzed. Careful testing of corneal sensation, avoidance of lower lid injections (to avoid ectropion), and treatment of epithelial defects are necessary. Use caution in angle closure glaucoma. Product contains albumin and may carry a remote risk of virus transmission (theoretical Creutzfeldt-Jakob disease via albumin component; no cases identified to date). Prior to treatment, a primary axillary hyperhidrosis evaluation for secondary causes (e.g. hyperthyroidism) should be made. When used for temporary reduction in glabellar lines, it should not be administered more frequently than every 3 months. Serious adverse events (including fatalities) in association with the use of botulinum types A and type B reported. The most serious outcomes, including respiratory failure and death, were associated with use in children for cerebral palsy limb spasticity. Should be administered with caution to patients with thrombocytopenia or bleeding disorder (e.g. hemophilia) or those receiving anticoagulant therapy. Patients who develop tolerance to botulinum toxin type A may respond to botulinum toxin type B or other botulinum toxin serotypes (e.g. botulinum toxin type F). Long-term effects of chronic therapy unknown.