I have used EFT for cancer. When the cancer is reversing the body does react. The patient becomes very tired and gets infections as the cancer breaks down. I did a patient with lung cancer (14 nodules), he became ill with "bronchitis" for 5 weeks and used Antibiotics . After he recovered X-rays showed no sign of cancer.
I actually use WHEE, which is much quicker and simpler that EFT. It also seems to have less reversals. Its great with teens as it doesn't look so weird and for older patients that have difficulty recalling the sequence.