Heroin

Heroin is diacetylmorphine, formula C21H23NO5, molar mass about 369.4 g/mol, CAS 561-27-3, PubChem CID 5462328. Chemically it is just morphine with two acetyl groups bolted on, and that small change is the whole story; the acetyl groups make the molecule far more fat-soluble, so it slips across the blood-brain barrier within seconds of reaching the bloodstream. Once inside, enzymes clip the acetyl groups back off, turning it into 6-monoacetylmorphine and then morphine, the compounds that actually do the work at opioid receptors.

That fast entry is why an injection or a smoke of heroin hits harder and quicker than the same amount of morphine, and it is a big part of why the drug is so strongly reinforcing. Bayer first sold it in 1898 as a cough remedy, marketed as a supposedly non-addictive morphine substitute, which turned out to be badly wrong. Today it has essentially no legal recreational status; it sits in Schedule I in the United States, while the same molecule is used as a tightly controlled medicine called diamorphine in the United Kingdom.

The headline risk with heroin is that it stops you breathing. Because mu-opioid agonism suppresses the brainstem respiratory centers, an overdose brings slow shallow breathing, pinpoint pupils, blue lips, unconsciousness, and death if nobody intervenes; it can be reversed if the opioid antagonist naloxone (Narcan) is given in time, which is why keeping naloxone on hand saves lives. The danger has grown far worse on the illicit market, because much of what is sold as heroin is now cut with or outright replaced by illicitly made fentanyl, which is roughly fifty times more potent by weight; the user has no reliable way to know the real dose, and a small miscalculation is fatal.

Combining heroin with other depressants (alcohol, benzodiazepines such as Xanax, gabapentin, or other opioids) stacks the respiratory risk and is a common way people die. Regular use builds tolerance and physical dependence quickly, and stopping brings a miserable but rarely fatal withdrawal (aches, cramps, diarrhea, sweating, insomnia, and intense craving). Injecting carries its own harms: abscesses, endocarditis, and, when equipment is shared, HIV and hepatitis C. In the United States it is a Schedule I controlled substance, meaning no accepted medical use there; the legal medical form, diamorphine, exists only in a handful of countries such as the United Kingdom.