The antipsychotic drug fluphenazine has a prolonged period of action when it is given by intramuscular injection, but not when it is given by intravenous injection.Suggest why this is the case.

精神抑制药氟奋乃定，采用肌肉注射的有一段迟的，但是采用静脉注射却没有，说明为什么出现这种情况。