Heroin (Diacetylmorphine): general

Heroin, also known as diacetylmorphine, is derived from the opium poppy plant [1]. It's a morphine derivative [2] that acts as a potent opioid receptor agonist once it's broken down in the body, first to 6-acetylmorphine and then to morphine [2].

The most important thing to know before anything else: as with opioids generally, opioids bind to receptors in the brain that are supposed to signal the body to keep breathing, and opioids interrupt those signals, which can slow breathing and eventually stop it entirely [3]. It is also preventable — opioid overdose death is preventable with basic life support and timely naloxone [4]. Have naloxone on hand, make sure someone else is present or nearby, and make sure they know how to use it.

The second thing: what you buy may not be what you think it is. Pills obtained from an informal source can look like a safer, regulated drug but may contain nonmedical opioids including fentanyl at lethal doses [3].

Heroin has no approved medical use in the United States and is Schedule I under the Controlled Substances Act [1]. Elsewhere — the United Kingdom, for instance — the same molecule is a legal prescription drug under the name diamorphine, used for severe pain, including postoperative and terminal care [1][2].

None of this tells you how much to take or how long it lasts — start low, don't use alone, and don't combine with anything else that also slows breathing.

References

1. Heroin (Archived) — NCBI Bookshelf (National Institutes of Health)
2. diamorphine | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY — IUPHAR/BPS Guide to Pharmacology
3. Opioid Overdose: Signs, Symptoms & Treatment — Cleveland Clinic
4. Opioid overdose — World Health Organization