Oxytocin is pretty hard to find online, but here is an apparent source in nasal spray form: http://www.oxytocinstore.com/oxytocinspray.php

While not really a castration drug, it apparently can induce lactation "letdown" (meaning if you already have prolactin-related lactation and breast development).

It also may have use in anxiety and fear management although the half-life is only about three minutes. It is normally produced in the body at orgasm, and may have a lot to do with the after-sex bonding feeling.

And here's another source: http://www.oxytocinfactor.com/how-to-bu ... in-factor/

I guess it is becoming more common now, because I've searched before without luck.