How do you overcome this disadvantage of placebo-controlled double blind clincial trial?

If a patients know his drug may work so there is a placebo effect, but in a clinical trial, a patient
also know his drug may be a placebo. Because the trial
is doubvle blind, so a patient on an active drug
think he has 1/2 chance to be duped by a palcebo,
this negatie effetiveness of an active drug.