This one seems to be very hard to find. Since most Gender Bender Hentais are more about feminization instead of masculinization. Even when I type masculinization in Google it shows me the feminization category. It was about 2 years ago when I saw that doujin.
It's about two Girls who are friends. One of the Girls got a potion that is able to turn one into the opposite sex. She put it in her Girfriends drink to see if it actually works. After she drank it nothing happened at first. As the day goes on she got aroused by seeing other girls, what was really weird to her. Later she grew a dick and starts masturbating and getting more masculine until she was no longer a woman anymore. Her Girlfriend was there witnessing the transformation and after that they had sex.