Suki da yo, Shinyuu!

Hibiki, a sales representative for a pharmaceutical company, has known his colleague Chiaki, a researcher, since their student days. They’re best friends who can talk openly about almost anything—except one secret Hibiki keeps to himself: he has feelings for Chiaki! But when Hibiki accidentally drinks one of Chiaki’s mysterious inventions, his hidden emotions start to spill out in a completely unexpected way...!

(Source: Ficus, translated)

Note: Includes 1 extra chapter.