Garrett has taken one substance that has a drug effect of 5, and a second drug with a drug effect of 4. if the combined effect is equivalent to 11, what is the combined drug effect called?

The right answer is Hyperadditivity.

Hyperadditivity, or synergism is a type of phenomenon in which several factors acting in common together create a global effect; a synergistic effect distinct from everything that could have happened if they had operated in isolation, either individually or together but working independently.