Drugs have a profound effect on the human nervous system. Opiates like heroin, Oxycontin, and morphine depress both the somatic and autonomous nervous system. Downers can make your skin temperature change, and generally affect all of your main senses. It can also depress your fight or flight response causing a delayed response to risk and danger.
Opiates can also cause the body to neglect autonomous responses like breathing. Your body literally forgets to tell itself to inhale. I have worked closely with drug addicts, and I have witnessed an overdose first hand (luckily doctors were able to revive that person with Narcan). It is not a pretty sight to see someone unable to breath, open her eyes, and literally fade out in front of you.