Final answer:
Antagonists are drugs that reduce neurotransmitter activity by receptor interference, while agonists increase activity by imitating neurotransmitters or enhancing their production.
Step-by-step explanation:
An antagonist is a type of drug that decreases the activity of certain neurotransmitters by blocking or interfering with their receptors. This action prevents neurotransmitters from binding to their receptors, reducing their effects. For instance, in treating diseases like Parkinson's disease, where dopamine levels are low, dopamine agonists are used to mimic the neurotransmitter, whereas antagonists would have a contrasting effect by blocking receptor activity, which can be either beneficial or harmful depending on the condition being treated. Drug interactions can be synergistic, where drugs enhance each other's effects, or antagonistic, where they reduce effectiveness or increase potential for toxicity.