TABLE 1.

Topical analgesia with opioid and opioid combinations

	Drug ED50, mM (95% conf limits) Ratio
	Lidocaine	Morphine	Meperidine	Methadone
Alone	2.3 (2, 3.4)	6.3 (3.7, 8.3)	1.8 (1.1, 2.4)	5.0 (3.7, 6.8)
Combinations
Lidocaine +	0.8 (0.6, 1.1)		0.7 (0.6, 0.9)
Meperidine	3		2.5
Lidocaine +	0.6(0.4, 0.7)			1.2 (0.9, 1.7)
Methadone	4			4.2
Morphine +		2.2 (1.2, 2.6)	0.72 (0.6, 0.8)
Meperidine		2.7	2.4
Morphine +		0.8 (0.6, 1.1)		1.6 (1.2, 2.1)
Methadone		7.6		3.2

ED₅₀ values were determined from dose-response curves and presented with 95% confidence limits. For lidocaine, the ED₅₀ value was determined only from the initial portion of the curve. Combinations were also examined using increasing doses of a fixed ratio of the indicated drugs. ED₅₀ values were determined and presented with the 95% confidence limits. The ratios of drug dose in combination to that of the drug alone are presented in bold. The relative potency of the various drugs in combination was compared with the same drug alone as a ratio. The fixed ratios were as follows: lidocaine/meperidine, 1; methadone/lidocaine, 2; morphine/meperidine, 3.5; methadone/morphine, 2