Jennifer has been depressed for several months, and she decided to take an overdose of sleeping pills. after taking the pills, her breathing rate decreased dramatically, her heart slowed down, and her blood pressure decreased. which brain structure is the drug acting on

The answer to this question would be medulla oblongata.

Medulla is part of the brain that will control autonomous nervous systems like breathing, heart rate, and blood pressure. Sleeping pills generally used to make the people relax and make the used heart rate, breathing and blood pressure reduced.