The analgesic efficacy and toxicity of oral diclofenac sodium 50 mg (q.i.d.) vs. nefopam 60 mg (q.i.d.) and a combination of 640 mg ASA and 40 mg codeine (q.i.d.) in cancer patients with moderate to severe chronic pain has been evaluated in a randomized double-blind study. Planned duration of treatment was 10 days. Pain intensity was evaluated by a visual analog scale. The length of patient participation in the trial, the patient's final global evaluation and the incidence of side effects were also evaluated. Ninety-nine patients were enrolled in the study. All treatments produced a statistically significant pain relief (P less than 0.01) without differences among groups but only 26 of 99 patients (26.3%) completed the planned treatment period. Mean time in the study was 4.65 days. Inefficacy and side effects were the main reasons for premature treatment interruption. Patients treated with nefopam had a significantly shorter period in the study than patients treated with the other 2 treatments. Adverse effects were slightly more frequent with the nefopam and ASA + codeine regimens. The 3 therapeutic regimens appear to be similar as to analgesic efficacy, but diclofenac presents the advantage of a slightly better safety profile than nefopam and the ASA + codeine combination.