A substance that inhibits the physiological effects of many illicit substances. Used in some addiction treatments such as NaltrexoneA drug prescribed to addicts to ease craving in the period following detox.. The opposite of an agonistThis is a drug capable of triggering the receptors in the brain, leading to an opioid effect. Heroin, morphine, and opium are prime examples of this category of drugs. There are forms of therapy focusing on this opioid response and designed to wean an individual of dependence..