"The drug was Phalanxifor, this molecule designed to attach itself to cancer cells and slow their growth." - Hazel Grace
"There is no such thing as Phalanxifor. I made it up, because I would like for it to exist." - John Green
Phalanxifor, as mentioned in the book, is a drug as a short term remedy of cancer that slows the growth of the cancer, but does not eradicate what is already there. It is an infamous drug for not working, in about 70% of people.
As it mentions in the end of the book, John Green states it is not a real drug.
In The Fault in Our Stars, the drug phalanxifor was taken by Hazel Grace to sustain her fairly normal level of cancer for a substantial period of time instead of letting it grow to higher and more dangerous stages.