Skiveren, J, et al. “Morphine Gel 0.3% Does Not Relieve Pain During Topical Photodynamic Therapy: A Randomized, Double-Blind, Placebo-Controlled Study”. Acta Dermato-Venereologica, vol. 86, no. 5, Sept. 2006, pp. 409-11, doi:10.2340/00015555-0128.