This is incorrect. See the correct answer by attorney Elman. This is potentially patentable, however your "I am sure" would need extensive testing. Still if you file a patent application and the medicine does have this unexpected result, you have potential to have a valuable patent. Get competent help from a patent attorney if you have this situation. Indeed, the new AIA (patent reform) places importance on such filing to be done quickly.

Of course, the first-inventor-to-file provisions of the AIA enacted in September 2011 will apply only to patent applications first disclosing inventions on or after March 2013.

Unless (remote chance) ruled unconstitutional, invalidated, or repealed prior to then.