In this case, the person feels better right and the improvement is ascribed to the drug. As time goes by the effect wears off and all sorts of explanations are entertained. What actually happened is that the placebo effect, which can be very powerful in the short term, loses its force. In fact, since antidepressants typically take two to four weeks to produce an effect ,the very promptness of improvement should alert one to the strong possibility of a placebo effect. Things are rarely simple.
People often have a placebo effect and a true drug effect at the same time.
In that case, improvement should be sustained. Or a drug that produces only a placebo response may bring about a true and sustained effect after a longer period of time or in a higher dose. There can even be a short placebo response, followed by a worsening, and then improvement as the drug kicks in. To complicate matters, some placebo responses can be enduring. Conversely, side effects and bad reactions may be negative placebo reactions rather than drug effects.