Glyceryl trinitrate (a.k.a. nitroglycerin) patches can cause severe headaches [1,2]. Why?
-  Bokhari, Ali R., and George AC Murrell. "The role of nitric oxide in tendon healing." Journal of Shoulder and Elbow Surgery 21, no. 2 (2012): 238-244. https://scholar.google.com/scholar?cluster=18309170375075579531&hl=en&as_sdt=0,22; http://sci-hub.cc/10.1016/j.jse.2011.11.001
-  https://en.wikipedia.org/w/index.php?title=Nitroglycerin_(drug)&oldid=713221881