As you probably know, nubain is a synthetic opioid. Its related to oxymorphone, and its analgesic potency is equivalent to morphine. It has both opiod agonist (analgesic properties) & antagonist properties (sometimes its prescribed to reverse side effects from another opiod analgesic like morphine or oxymorphone).
Although nalbuphine is not supposed to be as addictive as other opioids, you will still become dependant after prolonged use and experience withdrawal symptoms if you just stop taking it (abdominal cramps, nausea/vomitting, restlessness, anxiety, elevated temp).
I really don't know how to properly taper off of nubain, but you should try to find out (I'll try to find out too). But don't just stop taking it suddenly.