Heroin is chemically diacetylmorphine -- a morphine molecule which has been acetylated. By reacting morphine with acetic anhydride, which is essentially two acetyl groups, both -OH (alcohol) groups on the morphine molecule are replaced with acetyl groups, -COCH3.

The potency of heroin comes from the rate with which is can cross the blood-brain barrier. The acetylation allows it to cross into the brain substantially faster than morphine, intensifying the "rush". Once in the brain heroin is rapidly metabolized into morphine, and the subjective effects are no different from morphine.

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