Oppai Heart - Kanojo wa Kedamono Hatsujouki!: vol.1
Ryuya lives a peaceful school life with his friends. But one day, he and his friends accidentally feel a medicine juice. Once you feel it, you'll become sexually sensitive and get excited. The breasts of the girls who feel it will get bigger and they will need to have sex to cool down. Also, to make a vaccine, they will need to get pregnant. Ryuya starts having sex with the girls to cool them down and make a vaccine.