An example of an additive effect across drug classes would be:

• A. Using a PGA alone yields greater IOP reduction than using a BB alone
• B. Using a PGA together with a BB yields larger reduction than either alone
• C. Using two PGAs together yields the same as one PGA
• D. Adding a CAI after an AA reduces efficacy

Answer: (B)

Additive effect across drug classes means that when you combine medications with different mechanisms, their IOP-lowering effects sum to a greater reduction than either drug alone. A prostaglandin analog increases outflow, while a beta-blocker lowers aqueous humor production. Because they act through distinct pathways, using them together typically produces a larger drop in IOP than using either one by itself. The other statements don’t show this cross-class additivity: comparing monotherapies doesn’t demonstrate a combined effect; two drugs from the same class don’t provide an added benefit beyond redundancy; and while adding a second class often helps, the idea that it reduces efficacy wouldn’t reflect how additive pharmacology works.