Fentanyl and heroin are 2 structurally different molecules. Heroin is metabolized into morphine and 6-monoacetylmorphine. Fentanyl is related to the phenylpiperidine class of opioids (meperidine or Demerol is the prototype). Both are lipid-soluble (cross BBB rapidly) and act primarily at mu receptors (analgesia, respiratory depression, constipation). Fentanyl is 100 times as potent as morphine, while heroin is about twice as potent as morphine. Fentanyl has a much shorter duration of action than heroin, but has a longer plasma half-life. Lipid solubility, pharmacokinetics (what the body does to the drug), potency, and affinity for opioid receptors are probably all involved in how easy it is to become physically addicted. This is why Xanax is so addictive, while Ativan is much less so.
