The best laxative for terminal cancer patients treated with opioids still remains to be determined. This comparative study was conducted with the objective of determining treatment and cost efficiency for senna and lactulose in terminal cancer patients treated with opioids. The methodology used a randomized, open, parallel group design. The study was conducted in the Palliative Care Unit in one Madrid Health Care District. Ninety-one terminal cancer patients were randomized into two groups: A = treated with senna (starting with 0.4 mL daily), and B = treated with lactulose (starting with 15 mL daily) for a 27-day period. The main outcome measures were defecation-free intervals of 72 hr, days with defecation, general health status, and treatment cost. Laxative efficacy was analyzed through t test and analysis of variance. No difference was found between the laxatives in defecation-free intervals or in days with defecation. The final scores for general health status were similar in both groups. Given that the two treatments have similar efficacy and adverse effects, a recommendation is made for the use of senna because its cost is lower than lactulose.