4.Agosti R, Duke RK, Chrubasik JE, Chrubasik S. Effectiveness of Petasites hybridus preparations in the prophylaxis of migraine: a systematic review. Phytomedicine. 2006 Nov;13(9-10):743-6. Epub 2006 Sep 20. http://www.ncbi.nlm.nih.gov/pubmed/16987643
5.Lipton RB, Gobel H, Einhaupl KM, Wilks K, Mauskop A. Petasites hybridus root (butterbur) is an effective preventive treatment for migraine. Neurology. 2004 Dec 28;63(12):2240-4. http://www.ncbi.nlm.nih.gov/pubmed/15623680
6.Diener HC, Rahlfs VW, Danesch U. The first placebo-controlled trial of a special butterbur root extract for the prevention of migraine: reanalysis of efficacy criteria. Eur Neurol. 2004;51(2):89-97. http://www.ncbi.nlm.nih.gov/pubmed/14752215