The Plot is wrong 

I just saw this film, and you have the plot wrong. The girls don't seek out the My Son drug. Eiji puts it into their IVs, that are for an experiment in contraceptives by his mother. They do not know they have had the My Son drug put into them, and neither does the mother. That's what makes the film so disturbing, is that they didn't choose to be a part of the experimentation of My Son.