When it comes to injectable procedures, there are always certain risks involved. As different people reacts differently to Botox, in general most patients do not experience any side effects.
There are the rare few patients who will experience drooping of muscles (eyelids or eye brows) after the treatment, however the effects last only temporarily until the Botulinum Toxin wears off.
Common side effect such as bleeding around the injection sites, headache, flu-like symptoms and local pain may occur after the treatment. Do make sure to call your licensed medical practitioner for advise.