Crashing with a prometrium question

I am crashing with a question for my sister.

Her reg ob put her on prometrium to bring on her period as she has really long cycles. he said to take it for 12 days? and then she should get her period...well after 5 days she got her period. She tried calling her dr but he didn't call her back. Should she stop taking the prometrium? Shes not sure what to do.

I figured I would ask here since this board was so much help to me