ef ‧ fi ‧ ca ‧ cious /ˌefəˈkeɪʃəs◂, ˌefɪˈkeɪʃəs◂/ BrE AmE adjective formal
[ Date: 1500-1600 ; Language: Latin ; Origin: efficax , from efficere ; ⇨ ↑ effect 1 ]
working in the way you intended SYN effective :
an equally efficacious method of treatment