effectual,
efficacious,
effective marked by qualities giving the power to produce an intended effect; "written propaganda is less efficacious than the habits and prejudices...of the readers"-Aldous Huxley; "the medicine is efficacious in stopping a cough"
uneffective,
ineffective,
ineffectual lacking the ability or skill to perform effectively; inadequate; "an ineffective administration"; "inefficient workers"
powerfulness,
power possession of controlling influence; "the deterrent power of nuclear weapons"; "the power of his love saved her"; "his powerfulness was concealed by a gentle facade"
