A combination of two or more nootropics is called a “stack." Combining several compounds into a stack can have synergistic effects, so many people see better results with several nootropics than with one. However, there’s a large degree of individual variance in how people respond to different substances. Certain drugs and supplements can interact to cause negative or potentially dangerous side effects, so if you’re uncertain about the safety of a nootropic stack, talk with a medical professional beforehand.