What is a drug-drug interaction?

• A. One drug affecting its own pharmacological effect.
• B. One drug affecting the pharmacological/therapeutic effect of another, when co-administered.
• C. An interaction that only occurs in IV administration.
• D. A drug cannot interact with another.

Answer: (B)

A drug-drug interaction is when two or more drugs are used together and one changes the effect of the other, altering the therapeutic response or increasing adverse effects. This can happen through pharmacokinetic mechanisms (one drug changing the other's absorption, distribution, metabolism, or excretion) or pharmacodynamic mechanisms (drugs affecting the same receptor or physiological pathway, leading to add-on, synergistic, or antagonistic effects). It isn’t limited to IV administration and isn’t about a drug altering its own effect.

For example, one drug may inhibit liver enzymes that metabolize another drug, causing higher levels and greater effects or toxicity of the second drug. Conversely, two drugs with sedative effects together can deepen sedation more than either alone. So the statement that best defines a drug-drug interaction is that one drug affects the pharmacological/therapeutic effect of another when co-administered.