This multicenter trial examined the efficacy and safety of dextromethorphan (DM) as an enhancer of analgesia and modulator of opioid tolerance in cancer patients with pain. Eligible patients were randomized to slow-release morphine plus DM or slow-release morphine plus placebo. The initial DM dose was 60 mg four times daily for seven days, with an increase to 120 mg four times daily, if tolerated, for another seven days. During the study, patients recorded medications and scores for pain, nausea, drowsiness, and insomnia. Sixty-five patients were randomized. Although average pain scores (12.6 vs. 15.8), number of breakthrough doses (9 vs. 11.3), and change in total morphine consumption (550.9 mg vs. 597.1mg) were less in the DM group than placebo group, the differences were not statistically significant (P=0.31-0.33). Side-effect scores were not statistically significantly different. Dizziness was greater in the DM (58%) than placebo (36%) group. This study showed a statistically nonsignificant enhancement of analgesia or modulation of opioid tolerance in cancer patients with pain when DM was added to morphine. Participants receiving the DM also had more toxicity, particularly dizziness. This toxicity and the limited evidence of effect do not support the use of DM to enhance opioid analgesia or to modulate opioid tolerance in cancer patients.