3-Monoacetylmorphine or 3-acetylmorphine is less active metabolite

Metabolite

Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...

 of heroin (diacetylmorphine), the other two being morphine

Morphine

Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...

 and more active 6-monoacetylmorphine (6-MAM).

Because of the acetyl-group in 3-position, 3-MAM has relatively weak affinity to μ-opioid receptors.

As 3-O-acetylmorphine-6-O-sulfate (C₁₉H₂₃NO₇S), where 6-OH is changed to 6-O-SO₃, it can act as potent, centrally acting morphine derivative and has important analgesic properties.