Copypasta here, because I'm fucking proud of this!
Anyway a little tidbit on the history of heroin:
Heroin was the first designer drug. Morphine was first isolated in 1804, but only got popular in the american civil war. After 30 years of popularity, it got banned in a lot of countries.After that Bayer, using the same method as with aspirin(anhydrous acetic acid+salicylic acid+pressure), made heroin. It was marketed as a non-addictive substitute for morphine, but the molecule was only different from morphine in that the two -OH group(3- and 6-positions) got one-one acetyl substitute for the H atom. Heroin was usually taken orally and with that route of administration, it got quickly metabolized into morphine(O-deacetylation, same as 4-AcO-DMT), so it was basically morphine. When injected/smoked/snorted mainly the acetyl in the 3-position is deacetylated and the result is 6-monoacetylmorphine(6-MAM). The difference between morphine and 6-MAM is that it crosses the blood-brain barrier more quickly, so it gives a quicker, more powerful high.
Opiates have surprisingly few side-effects, the main ones are very strong addiction, small therapeutic window(ratio of effective dose and lethal dose), high tolerance and the most interesting IMO is the weakening of the immune system(this is why more heroin user have AIDS, than meth users). The last one is because morphine and 6-MAM are actually produced by your own body and they have a role in immunomodulation.
Pic is a heroin molecule(upper acetyl is the one in the 3-position).