Heroin is a drug called an opioid which is derived from the opium. Heroin is a super illicit version of Morphine and a lot more nasty for a variety of reasons. It is a CNS depressant meaning that depresses the CNS response to stimuli usually leaving people in a very relaxed state, however they ignore the fact that this also affect to their internal processes. One of the most dangerous areas is when Heroin slows down breathing and the heart to the point where it just stops completely, which if that happens can lead to coma and death. Additionally disorientation, slurred speech, and aggression are a very big part of Heroin. It also doesn't help that needles usually used to inject Heroin in IVDA patients can be contaminated and reused by bloodborne pathogens like HIV.
Overdoses are exceptionally dangerous because as stated they can cause the heart to go into Cardiac Arrest or the lungs to reach the point of Respritory Arrest. Because of the amount of overdoses on the drug EMS personell carry a drug called Narcan whovh is designed to be an Opioid antagonists and stops the drugs processes within the brain reversing the affects in as little as a couple of minutes.