I have 40 size bra,now I am a girl.how can I reduce my bust size urgently ?

There is NO medication you can take.
A couple of ways would be serious weight lose or invasive surgery.

Would you like to talk here about the urgency? Has something happened? How old are you and where do you live?

There is no pills or medication for this, if you see something advertised in magazines it is just a scam. Toning the muscles can help with issues of cup size to a point, What is happening to be such a problem