Why would Han Solo need to smuggle "spices"? What kind of "spices" are these...?

I mean, are they alien equivalents of our salt, pepper, cinnamon, etc... and if so WHY would they be illegal?!? Or are we talking drugs? But I can't see Han Solo smuggling the alien equvalent to cocaine and heroin, can you?... Have normal herbs and spices ever been illegal anywhere in real Earth history?