"Heroin has a (slightly) different chemical structure to Morphine, though it is more potent by weight."
If I remember correctly it crosses the blood brain barrier faster, but once it's in the brain it acts exactly the same as morphine. I've heard it's actually less potent than morphine gram for gram if you eat it, but I'm not sure.
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.
Orally, Heroin has a bioavailability of between ~25% and ~65%, depending on dose and tolerance, whereas Morphine has a bioavailability of 20-40%, making Heroin the more efficient of the pair.
If I remember correctly it crosses the blood brain barrier faster, but once it's in the brain it acts exactly the same as morphine. I've heard it's actually less potent than morphine gram for gram if you eat it, but I'm not sure.