Additivity is defined as

Additivity is defined as

• A. The combined effect equals the sum of individual effects
• B. The combined effect is greater than the sum of individual effects
• C. The combined effect is less than the sum
• D. No interaction between drugs

Answer: (A)

Additivity means the total effect of two drugs given together is the simple sum of their individual effects. This happens when they act independently, so their effects just stack up. For example, if one drug gives a 2-unit effect and the other a 3-unit effect, their combination would yield about 5 units. This is distinct from synergy, where the combination is greater than the sum, or antagonism, where it’s less than the sum. Saying there is no interaction is related, but the explicit definition of additivity is the arithmetic sum of the individual effects.

Additivity means the total effect of two drugs given together is the simple sum of their individual effects. This happens when they act independently, so their effects just stack up. For example, if one drug gives a 2-unit effect and the other a 3-unit effect, their combination would yield about 5 units. This is distinct from synergy, where the combination is greater than the sum, or antagonism, where it’s less than the sum. Saying there is no interaction is related, but the explicit definition of additivity is the arithmetic sum of the individual effects.