Prolactin is a hormone that is produced by the pituitary gland, a small gland located at the base of the brain. It is essential for the proper functioning of the reproductive system. Prolactin stimulates breast growth and milk production in women. In men, it affects sperm production and libido. Elevated prolactin levels may indicate a pituitary tumor and can cause menstrual irregularities and infertility in women, and decreased sex drive and erectile dysfunction in men.