morphine = long-acting, the active molecule in opium
fentanyl = short-acting, more potent synthetic morphine
Heroin was the first opioid derivative. It is supposedly less addictive than morphine. It is faster-acting and more potent than morphine.
Why are they so addictive? In many/most people, they induce an intense euphoria.
Part of the addiction mechanism is how it works on the nerve cell receptors. There are drugs other than morphine types (any MD’s or pharmacists please correct me) such as Toradol that effectively act on the opiate receptors but does not zone the person out and is not as addictive. This I know from some broken collarbones (personal experience), and I do not like how some painkillers zone me out.
Even if a person tries to tough out the pain, the pain also causes elevated blood pressure.
Toradol use still needs to be monitored, since it has some nasty side effects on the GI tract. It’s not good for any long term use. http://www.drugs.com/sfx/toradol-side-effects.html