What is the term for a medication that produces effects opposite to those intended?

The correct term for a medication that produces effects opposite to those intended is "antagonist." An antagonist is a type of drug that binds to a receptor and blocks or dampens the biological response that would normally be activated by an agonist. For example, if a medication is designed to lower blood pressure, an antagonist may prevent that effect, resulting in an increase in blood pressure instead.

In contrast, agonists are substances that activate receptors to produce a desired effect, whereas agonist-antagonists can both stimulate and inhibit effects depending on the situation or the particular receptors involved. Synergists are substances that work together to enhance the effects of each other, rather than producing opposing effects. This distinction makes the classification of antagonists important in pharmacology, particularly when it comes to understanding how certain medications can counteract the actions of others.