Six months later, Sam gives birth to a healthy baby girl. 

Sam is very happy to be a mother and is settling in to her new role well at first. Six months later, Sam is having difficulties with breastfeeding her baby. She tells the paediatrician that at first she was producing a lot of milk but she now thinks that her milk production is not enough to keep her baby well-fed. Sam tells the paediatrician that one of her friends was taking a drug to increase her milk supply and she would like her to prescribe this for her. 

The paediatrician reluctantly agrees. 

What is the mechanism of action of the drug that Sam was most likely prescribed?