Roy Hamilton of the Perelman School of Medicine says, “To our knowledge, tDCS [transcranial direct current stimulation] is pretty safe technology. It has not yet been associated with serious adverse effects.”
http://health.usnews.com/health-news/patient-advice/articles/2016-08-30/non-surgical-brain-stimulation-shows-promise-but-unknowns-remain U.S. News & World Report