Is it true that doctors take a cut every time they put a patient on a drug?
Doctors are actually under very strict anti-kickback laws. It would be illegal to take a payment from a drug manufacturer for writing a prescription. It's also illegal to refer a patient to a facility you have a financial interest in. This doesn't mean this sort of thing doesn't happen. Every once in a while I see an offer for some sketchy scheme where someone wants to pay doctors to prescribe something, usually a device, but I did see one for prescribing "pain creams". I think that one was a Medicare fraud: the outfit would charge Medicare for the device and pay the doc for the Rx. Getting caught up in Medicare fraud is a guaranteed way to be shut down not only by your state medical board, but the FBI!
Some people complain about drug reps bringing lunches and pens to doctors, and a lot of hospitals have prohibited the practice. Basic psychology shows that you are more likely to prescribe something you are more familiar with, or have heard about more recently. A few docs have made good money giving lectures on drugs to their peers or patients and getting speaker fees from drug companies. That's legal.