Despite the best efforts of certain legislators and the FDA, doctors still have the right to prescribe whatever drugs they think may help their patient. The danger for the doctor is that if someone DOES have a bad reaction or bad result, it leaves the doctor open for a malpractice suit. Something like metformin that has been proven to be very safe, and which a cancer patient might reasonably take for blood sugar issues, is a pretty safe prescription for patient and doctor.

I've had personal experience with taking a drug off-label so this is not some urban legend.