Seieki Tairyou Chuunyuu! 5 ~Ore no Takabisha na Bishoujo Goshujin-sama ga Konna ni Seidai ni Aheru Wake ga Nai!?~

A young man (protagonist) working his way though school as a rich family's house butler was mistreated daily by his high-soaring mistress and her first maid. He was butler in name, but in actuality his status was a lowly slave. One day. A small package arrived in the mail, addressed to the housemaster (the bitchy mistress's grandfather). Some sort of medicine...?

The protagonist peeked inside and found a special drug that promised to increase one's sperm load to 1 litre, with aphrodisiac effects. Who knew such a thing existed... The next day. Amid his misery, he noticed a timid maid who watched his suffering without intervening at all. He was convinced that in her heart, she too looked down on him. And in a fit of rage, he drank the aphrodisiac.

*Gulp*

In an instant his body flared with hot passion, and his groin bulged.
He unleashed a gigantic torrent of bukkake cum all over the maid!
Covered in his cock spittle, her personality changed... He took the newly aroused slut to a private room, and fucked her silly. He'll quell his burning revenge with the hormone sprays of a drug-powered super dick.

[From DLsite English]