Morphine becomes heroin (diacetylmorphine) upon replacing the naturally-occurring hydroxyl groups (-OH) with acetyl groups (-OCOCH3). This increases fat solubility, which allows it to cross the blood-brain barrier much more quickly than morphine.
The acetyl groups get removed in the brain, and the resulting morphine has the traditional effects. Since heroin includes the extra weight of acetyl groups, it's implied that heroin is less strong per gram than morphine, but is easier to deliver.
The acetyl groups get removed in the brain, and the resulting morphine has the traditional effects. Since heroin includes the extra weight of acetyl groups, it's implied that heroin is less strong per gram than morphine, but is easier to deliver.