Final answer:
Drugs are considered antagonistic when their combined effects are less effective than when taken separately. Antagonistic interactions often result in a decrease in therapeutic efficacy or increased potential for toxicity.
Step-by-step explanation:
If drugs are less effective when taken together than when each is taken separately, they are known as antagonistic. Antagonistic drug interactions can result in a decrease in therapeutic effectiveness and lead to decreased absorption, increased metabolism and elimination, or increased potential for toxicity.
For instance, if a patient takes antibacterials which require an acidic stomach environment together with antacids that raise stomach pH, the effectiveness of the antibacterials can be substantially reduced. Sympatholytic drugs, a type of antagonists, work by blocking the adrenergic receptors and preventing neurotransmitters like norepinephrine or epinephrine from binding.
This effect contrasts with synergistic interactions, where drugs enhance each other's effects. It's important to understand that agonists and antagonists can affect neurotransmitter systems, with agonists enhancing activity and antagonists impeding it.