Stimulants are drugs that increase your heart rate, breathing rate, and brain function. Some stimulants affect only a specific organ, such as the heart, lungs, brain, or nervous system. Epinephrine is a stimulant. It is given during cardiac arrest to make the heart beat. Other times, stimulants can cause unwanted side effects. For example, pseudoephedrine found in some cold medicines can increase the heart rate. Taber's Cyclopedic Medical Dictionary, 23rd ed. F.A. Davis Company; 2017. www.tabers.com/tabersonline/. Accessed January 26, 2017. Review Date 1/26/2017Updated by: Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.