Table 6 Experimental ED ₅₀ values, without and with naltrexone (NTX) 1 mg/kg i.p., naltrindole (NTI) 2 mg/kg i.p. and nor-BNI 3 mg/kg i.p. for the antinociceptive activity of the intraperitoneal coadministration of morphine with methadone, morphine with fentanyl, and morphine with tramadol, fentanyl with methadone, methadone with tramadol, and fentanyl with tramadol in the hot plate test of mice

From: Isobolographic analysis of the opioid-opioid interactions in a tonic and a phasic mouse model of induced nociceptive pain