An antagonist is medicine that thwarts the effects of another drug. An antagonist is a substance that blocks the effects of another specified drug; for example, naltrexone is an antagonist for opioids, and antabuse is an antagonist for alcohol. Antagonists are used in the treatment of addiction and alcoholism. However, this is unlikely to keep an alcoholic or addict clean or sober, as a 12 Step programme and therapy in the safety a treatment centre environment is the most successful way of treating alcoholism. Signals in the brain are carried by neurochemicals. A certain amount of a neurochemical needs to be present between two neurons in order for the signal to be carried. An agonist reduces the amount of neurochemicals required to carry the signal.
This obviously makes it easier for signals to get carried. An antagonist is a chemical that blocks the effect of an agonist and so dampens signals in the brain. A neuron actually requires a certain amount of activity in order to stay healthy, so long-term use of antagonists can lead to neuronal death.