Because people know they could be taking a placebo, then having undeniable effect (psychoactive, redness, soreness, etc.) tells them that they are not using the placebo. Conversely, having no undeniable side effects lets them know that they are using the placebo. Thus, placebo usage is undermined.
"Active placebo" has an undeniable effect, but not the compound being tested, so that the test subjects cannot tell if they have the placebo or the real things.
Because people know they could be taking a placebo, then having undeniable effect (psychoactive, redness, soreness, etc.) tells them that they are not using the placebo. Conversely, having no undeniable side effects lets them know that they are using the placebo. Thus, placebo usage is undermined.
"Active placebo" has an undeniable effect, but not the compound being tested, so that the test subjects cannot tell if they have the placebo or the real things.