When might a synergistic interaction be dangerous?

The correct answer highlights that a synergistic interaction between substances becomes dangerous when the combined effect of those substances exceeds what is considered safe. In pharmacology, a synergistic effect occurs when two or more drugs enhance each other's effects, leading to a greater overall impact than what would be expected from their individual effects. This can pose significant risks, especially when the total effect surpasses the established safety threshold, potentially resulting in severe adverse reactions, toxicity, or overdose.

In contrast, the other options do not adequately capture the essence of a dangerous synergistic interaction. While it is important to note that some drugs may be considered safe at individual doses or that one drug may lack effectiveness alone, these factors do not inherently reflect the risk posed by their combined effects. Moreover, the absence of a therapeutic index does not automatically correlate with danger from a synergistic interaction, as many substances might still demonstrate safety under specific conditions even without a clearly defined therapeutic index. Understanding the dynamics of drug interactions is crucial for ensuring patient safety in clinical settings.