That is b/c oxymorphone has terrible oral BA. The only way to get anything from these is to either snort or inject them. Now when you say you are taking Norco, how many mg are we talking? Because you may have been able to handle taking a whole 10 mg orally. But when you go nasal the BA goes up to like 70-80% I think. Then IV is obviously 100%. So you really need to let us know what dose of hydrocodone you are taking in order for us to be able to even guess how much of that you should snort or IV.
In fact in the interest of harm reduction I would recommend you not use any of this until we can help confirm that you in fact opiate tolerant, because it can be very easy to OD on this stuff if you are opiate naive. Also keep in mind, tolerance rises very fast with these, and the WD from them is strangely immediate. Like you can take just one dose of this stuff, and have a nasty hangover/WD the entire next day, but it only lasts for one day. That is if you only took one dose. You said how they made you sweaty, that is why.
