We can rarely, if ever, be 100% certain about the effects of treatments.
Fair comparisons of treatments can provide a basis for being confident about the chances of beneficial and harmful effects of treatments. However, it is rarely, if ever, possible to be 100% certain about the size of treatment effects or to know exactly what will happen if a treatment is used.
BEWARE when someone suggests that it is certain that a treatment works or is safe.
REMEMBER: There is some uncertainty about the effects of nearly all treatments.