A substance is defined as any:

A) drug that has psychedelic effects.
B) over-the-counter prescription drug.
C) product that could potentially create an addiction.
D) natural or synthesized product that has psychoactive effects.

A substance is defined as any natural or synthesized product that has psychoactive effects. This definition is broad enough to include drugs with psychedelic effects, over-the-counter prescription drugs, and potentially addiction-forming products, but is not limited to these specific categories.