"In the case of the antiviral drug ritonavir, not only was one polymorph virtually inactive compared to the alternative crystal form, but the inactive polymorph was subsequently found to convert the active polymorph into the inactive form on contact, due to its lower energy and greater stability making spontaneous interconversion energetically favourable. Even a speck of the lower energy polymorph could convert large stockpiles of ritonivir into the medically useless inactive polymorph, and this caused major issues with production which ultimately were only solved by reformulating the medicine into gelcaps and tablets, rather than the original capsules."
http://en.m.wikipedia.org/wiki/Polymorphism_(materials_science)
http://en.m.wikipedia.org/wiki/Polymorphism_(materials_science)