I m a Muslim girl, I want to know if it is possible to apply the fat pork on my back as a medicine?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is not permissible to treat yourself with impure medicine like pork fat as the Prophet said: "Do not treat yourselves with what is forbidden." [Abu Daawood]  That is whether this treatment is by consuming the medicine, drinking it or applying it as a cream. So it is not permissible to apply pork fat on your body unless it is for a necessity and there is no permissible substitute to replace it.  Allaah Says (which means): {While He has explained in detail to you what He has forbidden you, excepting that to which you are compelled.}[Quran 6:119].

A person who anoints his back with pork fat or other impure items; this does not nullify his ablution but his prayer is not valid until he washes the place of his body that is impure.

Allaah Knows best.