Soon after taking a psychoactive drug, zachary experienced a diminished appetite, an increased heart rate, dilated pupils, and feelings of self-confidence and euphoria. zachary most likely experienced the effects of

I believe the answer is: Heroin

Heroin is known for its euphoric effect, which make it a really popular recreational drugs even though its usage is heavily regulated. The increase in heart rate happen because heroin stimulate the user's central nervous system to mimic its function during high-adrenaline/exciting situations.