1. NMDA-Receptor Antagonists and Opioid Receptor Interactions as Related to Analgesia and Tolerance 
Donald D Price, PhD, David J Mayer, PhD, Jianren Mao, PhD, Frank S Caruso, PhD 
Journal of Pain and Symptom Management 
Volume 19, Issue 1, Pages 7-11 (January 2000)
DOI: /S (99)

2. Fig. 1
Oral administration of MS 32 mg/kg with DM 8 to 64 mg/kg twice daily for 8 days. Animals receiving MS alone showed development of tolerance (rapid decline in maximal analgesic effect after day 3), while animals receiving MS:DM in ratios of 2:1, 1:1, and 1:2 remained responsive. In particular, MS:DM 1:1 appeared to be the optimal ratio for prevention of tolerance. Reproduced from Mao et al. (10) with permission
Journal of Pain and Symptom Management  , 7-11DOI: ( /S (99) )

3. Fig. 2
Potentation of MS analgesia by DM following a single dose of the combination. Maximal analgesia with the combination was significantly greater than MS alone during the first 90 minutes after administration (P < 0.05). Reproduced from Mao et al. (10) with permission
Journal of Pain and Symptom Management  , 7-11DOI: ( /S (99) )