Feminization Drug

The Feminization Drug is a special drug developed by Mihari Oyama with help from Chitose Azuma. It was designed to transform Mahiro Oyama into a middle-school aged girl for a prolonged period of time as part of Mihari's "Onii-chan Rehabilitation Project", after which additional doses of the drug would be required for Mahiro to remain female.

Origins
Mihari Oyama has always thought the world of her older brother Mahiro, and the two siblings were incredibly close as kids. The two always bathed together, Mahiro won Mihari stuffed animals at amusement park games, he bought her stickers when she became interested in them, and he even gave her rewards, like her signature hairclip, for doing well on tests. The latter in particular pushed Mihari to excel so she would receive more love and affection for her brother, and she soon developed into an absolute prodigy at everything she did. She broke records in middle school sporting events that remain unbroken to this day, and eventually ended up repeatedly at the top of her class.

Unfortunately, Mihari's success had a detrimental effect on Mahiro. Due to her excelling despite being younger than him, people would start to compare his lack of success to Mihari's success, which aggravated an inferiority complex that he felt in comparison to her. As a result, Mahiro stopped making time for Mihari and began isolating himself in his room, doing nothing but reading hentai manga and playing video games. He even stopped leaving the house altogether after a certain point.

On Mihari's first day of high school, she was delighted by how she looked in her uniform and went to Mahiro's room to get him to see her in it. By this point he was already isolating himself with a 'KEEP OUT' sign on the door, and received no response when she tried to talk to him. Mihari started to get worried about Mahiro, as she missed him and wanted to find a way to help him.

While walking to school, Kaede made an offhand remark to Mihari about how using makeup and wearing fashionable outfits can make someone look good and improve their self confidence. This gave Mihari the idea to turn Mahiro into a girl. She then realized that if there was no opportunity for progress, there would be no results. Thus, she got the idea to de-age Mahiro as well. From that moment, her goal was clear: to turn Mahiro into a young girl to reintegrate him into society through her "Onii-chan Rehabilitation Project".

Realizing that she needed funding and research in order to do this, she took several tests that allowed her to skip high school and go straight to college. There, she focused on biology and chemistry where she started the project by developing a drug that would turn Mahiro into a young girl.

Mihari failed to do it by herself, but the project eventually caught the eye of Mihari's faculty advisor Chitose Azuma, who made key contributions to perfecting the drug. Although Mihari believed that she did this out of the goodness of her heart, in actuality Chitose has a secret agenda that she wants to see carried out through the project as well.

The night before the events of "Mahiro's Confusing New Body", Mihari slipped the drug into Mahiro's food (drink in the anime) to commence with the project. That night as Mahiro slept, the drug transformed him into a middle-school aged girl.

Overview
The Feminization Drug is a liquid substance that is designed to de-age the male human body and transform it from male to female. Only a small amount needs to be ingested for the drug to take effect, and as such the drug is contained into tiny bottles identical to ones that contain energy drink shots. Mahiro suspected that if a larger dose was taken that she could've de-aged even further, though this hasn't been verified by Mihari.

The exact process of the transformation is unknown, but it is known that the drug takes a few hours for the effects to fully activate. The first sign of the drug working is an inability to get erections, as shown when Mahiro tried to indulge her comrade, twice, after taking additional doses of the drug. This is likely due to the fact that the body is processing that area as a vestigial organ that will soon disappear.

By the time Mahiro woke up the morning after ingesting the drug for the first time, his body had transformed completely. Mahiro had grown shorter than Mihari, his hair short hair had grown much longer, and he had been transformed into a teenage girl, roughly 13 years of age, that was in the early stages of puberty. As a result of this, Mahiro had smaller hands, a cuter face, a skinnier frame, a higher voice, small breasts, and fully functional female sex organs. Also, due to these new changes, Mahiro was unable to become aroused by the eroge games that she was previously addicted to, much to her frustration. Also, she found herself getting aroused by a BL (boy-love) game that she bought once by accident and tried to play to take her mind off it, much to her horror. Mahiro was also female enough to start getting her period on a monthly basis like an ordinary girl. However, she is not 100% biologically female, as she will turn back to normal once the drug completely wears off.

Despite the body de-aging to a previous point, it does nothing to put the user back into a previous state when they were in shape. Mahiro was aged back down to being roughly 13 years old, but she was still as out-of-shape as she had been before her transformation, as evidenced by her lack of stamina. However, the user ages like an ordinary girl, which eventually Mahiro's bust to grow enough to allow her to start wearing full-cup bras by her second year of middle school.

Mihari said that Mahiro would turn back to normal once the drug was out of her system, but the drug remains in the body for several months before it completely leaves. Thus, the user is forced to remain female until the time that the drug wears off. The first sign of the drug wearing off is when the user's "comrade" regrows. Mahiro felt uneasy for several hours before it regrew the first time, but successive times have shown it as growing back suddenly, without any prior warning. Mahiro typically feels a twinge between her legs once it happens. Once the "comrade" regrows, it becomes fully functional right away as shown by the fact that Mahiro started getting erections again right away. This has led to many comedic situations due to Mahiro needing to hide it until she can get another dosage of the drug, despite being in many situations where it would be extremely easy to see. Thus, Mihari developed a special fast-acting variant that would exit the body in an hour, as opposed to several months, for emergencies. The user also begins to regain erotic feelings towards girls during this time.

When additional doses of the drug are taken before it wears off completely, the user remains female, but also does not de-age again. These additional doses rather allow the user to remain female and continue growing up as they are. It is unknown what will happen if the drug wear off completely, but Mihari mentioned that it would wear off by morning when Mahiro's "comrade" regrew for the first time in the evening.

There is also a way that the effects of the drug can be temporarily halted, as shown when the user falls ill. When Mahiro caught a cold, her "comrade" temporarily grew back until she started feeling better. Afterwards, it disappeared and the effects of the drug continued. Mihari was at a loss as to why this happened, but Mahiro offered her own theory. She suspected that when she fell ill, her body's male reproductive instinct took over as a defense mechanism, causing her comrade to regrow temporarily. She compared it to how a guy gets an erection when he's stressed out or tired. She thought about recreating the experiment by getting sick via Miyo's special energy drink, but she ultimately decided against it.

Although designed for usage on a biological male, the drug was shown to have an interesting effect on a real female as shown when Nayuta accidently consumed the fast-acting variant thinking it was an energy shot. When consumed by a real female, their bust will grow tremendously by several cup-sizes. Even a girl like Nayuta who is flat-chested will develop and extremely large bust that surpasses even Miyo's bust in size. When Mihari, who was already full developed, took it, her bust grew so large that she looked like a starlet. These effects will wear off when the drug leaves the body.

History
In "Mahiro's Confusing New Body", Mahiro wakes up and realizes that he's changed into a girl, and realizes that Mihari was responsible after she confirms it. Mihari tells her that the effects of the drug won't wear off for awhile, and encourages Mahiro to enjoy living as a girl for a bit.

In "Mahiro's Panic! At the Hot Spring (Part 1)", Mahiro starts to feel the effects of the drug wearing off as she feels ill and feels a recurring twinge throughout her body. When she's in the hot spring with Momiji behind her, her "comrade" regrows due to the drug wearing off.

In "Mahiro's Panic! At the Hot Spring (Part 2)", Mahiro escapes the hot spring with Mihari's help, and Mihari explains that the drug is starting to wear off. Fortunately, she brought an additional dosage of the drug, and Mahiro drinks it without hesitating (in the anime she waits for a few hours to consider it, eventually drinking it after deciding that she loves her new life too much to give it up). Despite Mihari's teasing, she insists that it's not because she wants to be a girl, but rather because she wants things to stay as they are for now. Despite this, Mahiro's "comrade" fails to go away immediately, and Mahiro hides it from a suspicious Momiji. As seen in "Mahiro and the Comrade's Return", during dinner, Mahiro goes into the bathroom, and attempts to indulge her "comrade" before it vanishes again, but fails due to the effects of the drug. While everyone is asleep, Momiji's suspicions reach a peak, and she checks Mahiro's crotch to see if Mahiro is a guy. She finds nothing due to the drug's effects kicking in, and Mahiro is relieved that she is once again fully female.

In "Mahiro and Swim Class (Part 1)", Mahiro's "comrade" regrows while she's in the pool during swim class.

In "Mahiro and Swim Class (Part 2)", Mahiro fakes having her period to get out of the pool, and has Rin get a towel so she can cover up her private area. Nayuta helps her in the locker room, and eventually tucks and tapes Mahiro's comrade back with duck tape to get her through the rest of the day. Mahiro eventually leaves early after realizing in horror that she's getting dirty thoughts towards her friends, and rushes home before something else can happen. She bursts through the door and demands that Mihari give her an additional dosage of the drug, which she does. Mahiro takes the dosage and feels relieved, but then realizes in horror that she just subjected herself to more time being female.

In "Mahiro and the Comrade's Return Again", Mahiro realizes that her "comrade" is still there due to the drug's effects kicking in fully overnight. She decides to indulge herself a bit and boots up an eroge game. However, her comrade fails to respond, and Mahiro is seen later crying under the covers. Mihari tries to comfort her and explains that it was the effects of the drug.

In "Mahiro's Beach Fun (Part 1)", Mihari reveals that she has developed a fast-acting variant of the drug for emergencies and brought it along, saying that its effects will last an hour before wearing off. After being shocked by the sight of Miyo in a bikini, Mihari accidently drops the dosage into Nayuta's bag without realizing it. Later, Nayuta finds in while going through her bag, and, thinking it's an energy shot that Chitose put in, drinks it, before seeing the transgender symbol on the bottle in horror.

In "Mahiro's Beach Fun (Part 2)", Mahiro finds Nayuta, and learns that the effects of the drug have caused Nayuta's bust to grow tremendously. The two work with Mihari to keep it hidden until the effects of the drug wear off in an hour. After it is nearly exposed, the drug wears off and Nayuta's bust deflates back to normal. They claim that Nayuta was wearing a trick top to throw off any suspicions. Later at home, Mihari is surprised by the reaction that the drug had on a regular female, and decides to try it on herself "for science". Taking the drug causes Mihari's bust to grow extremely large, and she admired her reflection in the mirror. Mahiro sees her, then turns away in disbelief that Mihari would actually do this.

Some time after this, Mihari gave Nayuta a bottle of the fast-acting variant to give to Mahiro in the event of another emergency.

Trivia

 * This drug is one of two special drugs developed by Mihari for Mahiro, the other being the Warming Drug.