Table 5.

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 writhing test of mice

Drugs	Experimental ED 50
	Alone	+NTX	+NTI	+ nor-BNI
	(n = 24)	(n = 24)	(n = 24)	(n = 24)
Methadone/fentanyl	0.019	0.048*	0.21*	0.018
Morphine/Methadone	0.023	0.095*	0.089*	0.019
Morphine/fentanyl	0.034	0.061*	0.058*	0.027
Morphine/tramadol	0.811	1.05*	1.10*	0.752
Fentanyl/tramadol	1.023	1.27*	1.31*	1.010
Methadone/tramadol	1.032	1.25*	1.32*	1.024

*P < 0.05 compared with ED₅₀ experimental alone.