Distinguish between additive and synergistic pharmacologic interactions with examples.

• A. Additive: equals the product of individual effects; synergistic equals the sum.
• B. Additive: the sum of individual effects; synergistic greater than the sum.
• C. Additive: less than the sum; synergistic greater than the sum.
• D. Additive: equals the sum; synergistic equals the sum.

Answer: (B)

When drugs interact, you’re looking at how their effects add up. Additive interactions mean the total effect is simply the sum of each drug’s individual effect. If Drug A provides a certain amount of relief and Drug B provides the same kind of relief, you’d expect their combined effect to be about the sum of those two amounts. For example, if each drug lowers blood pressure by about 10 mmHg, together you’d expect roughly a 20 mmHg drop.

Synergistic interactions, on the other hand, mean the combination produces more than the sum of the parts. The drugs enhance each other’s effects, so the total impact is greater than what you’d predict by adding them. For instance, two analgesics with different mechanisms might give much stronger pain relief together than the sum of their separate effects.

So the correct description is that additive means the sum of the individual effects, while synergistic means the total effect is greater than that sum. An example of synergy is a non-opioid plus an opioid providing disproportionately more pain relief than either alone; an additive case would be two drugs each contributing a straightforward, predictable amount of relief that adds up.