Botox is injected approximately every 12 weeks. Side effects are few and temporary, and they include droopiness of the eyelids or eyebrows, discomfort for a day or two after the injections, and weakness of neck muscles. Side effects peak at 2 to 4 weeks and improve gradually and are usually gone by 8 weeks.
Overall, 70% of patients achieve 50% or fewer headaches a month with Botox. Thirty percent do not improve. Another 30% within the 70% achieve reduction of 75% or more per month of headaches.