In the Miss Kobayashi's Dragon Maid manga it is stated by Tooru that eating her tail cures all ailments, but I have a very hard time believing that is all it does. In the anime Tooru sometimes thinks about the possibility that Kobayashi will eventually die despite the fact that Tooru has implied in the manga that she has access to some kind of fruit that makes humans immortal. Fafnir also told Shouta in chapter 14 of the Lucoa spinoff manga that bathing in his blood would make Shouta immortal.
I'm asking hoping that someone with a ton of knowledge about dragon mythology could point me to a reference or something regarding the effects of eating part of a dragon. I've read pretty much everything related to the series except some of the Elma manga. I can't find one instance where the tail thing is explained aside from the one chapter where Tooru says it cures illness. I'm pretty sure Fafnir was an old European myth in real life, and maybe some of the other dragons are too. It might be possible to figure out the writer's intent if one knows the material that inspired some of the characters.