I don't think there is any evidence for that. Dried opium is 10% morphine. If you take 10 mg of morphine or 100 mg of opium, it makes no difference, you're still getting the same amount of an addictive drug.
It does make a difference - opium is comprised of many alkaloids, in a similar way to how cannabis is made up of many cannabinoids. These alkaloids increase or decrease the effects of each other, as well as exerting their own influence.
> Opium has alkaloids that can cause severe nausea, etc.
Yes, it's well known that the alkaloid morphine causes nausea[1]. I can tolerate up to 30mg of immediate-release morphine (prescribed!), but beyond that I get bad nausea that is seriously off-putting. The nausea would certainly be a barrier to me getting addicted!
