From PLoS ONE

Placebos administered without deception may be an effective treatment for IBS [Irritable Bowel Syndrome]. Further research is warranted in IBS, and perhaps other conditions, to elucidate whether physicians can benefit patients using placebos consistent with informed consent.*

Source the article here.

* Kaptchuk TJ, Friedlander E, Kelley JM, Sanchez MN, Kokkotou E, et al. (2010) Placebos without Deception: A Randomized Controlled Trial in Irritable Bowel Syndrome. PLoS ONE 5(12): e15591. doi:10.1371/journal.pone.0015591