Banished from the Hero's Party, I Decided to Live a Quiet Life in the Countryside, Vol. 4 (manga) — Zappon

“Apothecaries have a lot of free time on their hands, huh?”
“In that case, let's just forget about work and relax!”
And just like that, Red and Rit decide to play hooky from their shop to go on a riverside date, where --
“hey, let's swim!”

Quality time with a princess in a swimsuit proves to be a much-needed balm to our protagonist's heart. But not all is fun and games, as the rapid proliferation of a dangerous drug among the populace of sleepy Zoltan leads to a rash of disturbing, violent incidents...