intranasal is IIRC about 40-60% effective, and I was an idiot and IV'd them and got severely hooked on opanas a couple years ago to the point where I'd set out lines the night before, fluffed up a few hundred milligrams (I had a monster tolerance from all the OC 80s I had been doing before) with 20mg per line on my bedside table and I couldn't even open my eyes in the morning without excruciating painful withdrawals if I didn't snort some instantly so I was CONSTANTLY havin to think about my next dose. I HATED that about oxymorphone, but it was an extremely potent analgesic and it felt stronger than heroin, and the numbers say it's much more potent IIRC... Especially factoring street dope bags vs pharmaceutical quality and quantity assurance.....
During this time, the Opana ER were of the octagonal variety / not abuse proof at all, nor were they made of an overly hard extremely dense plastic such as the reforumulated oxycontins, these were the ones that would easily crush and everyone I knew started to use them after I lost access to my old formulation OC's all those years ago, and I switched to opana IR and ER and it seemed like thousands of people did the same switch, or to dilaudid 8mg. At least those are easy to dose and less of a threat for parenteral use anyways. IV oxymorphone seriously was the most intense addiction I've ever overcome. coming off of it was worse than fentanyl....
One thing that really got me was how short using just a few consecutive days of dosing and it's impact..... it had a enormous effect on my tolerance. I never mixed or felt the need to mix anything with it.