Relistor is a modified form of naltrexone (methylnaltrexone to be exact)and it is a narcotic antagonist. Unlike other narcotic antagonists like Narcan, methylnaltrexone will not reverse central nervous effects of opiates such as analgesia and respiratory depression.
The itch you describe is usually associated with the opiate (hydrocodone) and not Relistor.
As far as the dosing is concerned, I think the commonly prescribed 8mg dose is insufficient---12mg is more like it.
There is no such dose of hydrocodone/apap as 20-325. Vicodin is now supplied in fixed combinations of 5/300, 7.5/300 and 10/300
Other analgesics like Norco and Anexsia come in similar combinations with slightly more acetaminophen (325mg vs 300mg)
Do you mind if I ask what you do for a living? You’re very knowledgeable.