The greatest medical ninja should have learned it somewhere.
Tsunade's skills extend beyond the typical medical ninjutsu. She earned fame during the Second Shinobi World War for being the only person able to make antidotes for the poisons of Sunagakure's Chiyo. Similarly, she can make sleeping potions that are undetectable to even the most skilled ninja. She is knowledgeable enough to perform autopsies and examine damage on a cellular level.
Source from Narutopedia.