> 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!
In fact, pure morphine has fewer side effects than opium. Opium has alkaloids that can cause severe nausea, etc.