Partial opioid agonist
A partial opioid agonist is an opioid with a lower maximal efficacy which makes it unable to produce as large an effect as a full opioid agonist even if the dose is increased. Suboxone.com defines a partial opioid agonist as, "a drug or medication that can both activate and block opioid receptors, depending on the clinical situation. Under appropriate conditions, partial agonists can produce effects similar to those of either agonists or antagonists."
Examples of partial opioid agonists: buprenorphine, butorphanol, nalbuphine, pentazocine