Four hundred eighty-eight BTX-A-naive participants were included in the double-blind trial and randomly divided into placebo ( n = 122 ) , low-dose ( n = 183 ) , and high-dose ( n = 183 ) treatment groups for injection with saline solution , 10 units and 20 units of HBTX-A , respectively , at 4 sites in the corrugator muscle and 1 site in the procerus muscle .