Bowel cancer specialists face an ethical dilemma when deciding to mention drugs to patients not endorsed by NICE and not routinely available on the NHS.
Nine out of ten doctors have revealed that they feel uncomfortable when discussing such treatment options with their patients.
As a result, over half have refrained from mentioning all potential treatments to patients with bowel cancer.
Patient group Beating Bowel Cancer says this can deprive patients of the right to seek funding for treatment.
Beating Bowel Cancer are calling for immediate guidance to be sent to general practices, to help guide health professionals dealing with patients who have the disease.
Beating Bowel Cancer Chief Executive Hilary Whittaker said: "Although we empathise with doctors who don't want to raise patients' hopes, we are aware, through speaking with patients, that the vast majority want to be told of all clinically proven treatments appropriate to their condition.
"We want to be reassured that bowel cancer specialists are provided with whatever support they require in order for such conversations with their patients becoming accepted as normal practice."