The following prayer only  works for Christians.

Prayer for Healing Cancer:
"In the name of Jesus Christ of Nazareth, I neutralize all carcinogens in (this person) right now. For any chemo given to (this person), may it have no harmful side effects but only do what the doctors intended it to do. 

I speak death to all cancerous and pre-cancerous cells in her body right now, that they would die as befits righteousness.  Heavenly Father, I ask you to flush these dead cells out of her body and heal the damage they have done.

Heavenly Father, please repair the DNA damage the radiation has done to her body. May (this person) grow in wisdom and love. May (this person) grow in faith and hope. May (this person) be blessed in all she undertakes.

I break any generational curses of cancer upon this family, in Jesus' name.  I break the empowerment and assignments given any demons on (this person), their property and environments, especially idols and charms.  I command all evil influences to leave these right now. 

For anyone who would work or conspire against (this person), I bind any unclean spirits over them to silence & impotence and I ask they be drawn toward salvation in all areas of their lives.

I ask this in Jesus' holy name, Amen."