The most common side effect is temporary bruising at one or more of the injection sites. This can be covered with make-up. On occasion, the Botox can migrate (leak) into nearby muscles, causing temporary weakness. There are rare cases of drooping of an eyelid or asymmetry of facial expression. The chance of experiencing a side effect depends which muscles are injected. Any side effect is temporary, as the effects of Botox are completely reversible.