“ConclusionsThese data suggest that low-efficacy opioids possessing significant mu-agonist activity (e.g. buprenorphine, butorphanol, nalbuphine , (−)-pentazocine) may potentiate the effects of cocaine in a manner similar to that typically observed with high-efficacy mu opioids.”
“The high-efficacy mu opioid levorphanol, and the low-efficacy opioids buprenorphine, butorphanol, nalbuphine and (−)-pentazocine, dose-dependently enhanced the effects of cocaine at doses that did not alter locomotor activity when administered alone.”