Dark or grimdark fantasy with the main character being a former soldier/mercenary in a past war and having to consume dangerous substances.

[Daphne]

The setting of the book was about a hero that had to become a substance using soldier similar to how Geralt of The Witcher books does in a fantasy setting. Him or another character in the book might have been a dark elf but it's not something that might be absolutely sure though. The book was from the modern age from 2000 to 2017. I tried searching in the database but it appears to be one of the less famous books.

[Kris]

Δάφνη, can you tell us more about these dangerous substances, what they look like and their effects? How and why does he consume them?

[Daphne]

The protagonist used substances in the form of potions and alchemical concoctions. Their effects had been to make the protagonist's body strong enough to fight in a conflict for an empire that used him as a tool of war, but their side effects gave him extreme pain for every time they had been used.