the present invention provides orally administered pharmaceutical compositions which contains an effective amount of free base or pharmaceutcially acceptable salts of nalbuphine and / or nalbuphine ester , an oily substance , and a solubility - assisting agent . the oily substance is preferably sesame oil . the solubility - assisting agent is preferably benzyl benzoate . the pharmaceutical composition is useful as an analgesic . the compositions achieves a much higher bioavailability rate and yields much longer lasting effects on nalbuphine than other nalbuphine products currently in the market .