Narcotic antsgonists. Naloxone is capable of blocking placebo response since both pain relievers such as morphine and endorphins from placebo both work on mu pain receptors. Naloxone is short acting, and usually administered I.V. And/or i.M. Naltrexone is an oral antagonist which works by a different mechanism.
Unknown. Since some large part of the "placebo" response is the bodies' own release of endorphin, it makes sense to think that the use of a blocking agent might interfere with that. Naloxone is very short-acting and is usually used intravenously (and sometimes thru nasal spray), but Naltrexone (and nalmefine) are oral drugs that last much longer. None of my patients on this have complained of pain.