Ive got polysethymia vera, how often can I extract blood ?

What you will need is an order from your doctor to have a theutic phlebotomy, when you get one you will go to a lab and they will run a hemoblobin and hematocrit on you and if it is high enough they will remove 1 to 2 units of blood from you. That will drop your hemoglobin down. You will probably have to have this done every couple of months for the rest of your life.