J. Skiveren, M. Haedersdal, P. Philipsen, S. Wiegell, and H. Wulf, “Morphine gel 0.3% does not relieve pain during topical photodynamic therapy: a randomized, double-blind, placebo-controlled study”, Acta Derm Venereol, vol. 86, no. 5, pp. 409–411, Sep. 2006.