Ibrance (Palbociclib) is a selective inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6) that is used to treat advanced or metastatic breast cancer in postmenopausal women and men. By blocking CDK4/6, Ibrance slows the growth and division of cancer cells, making it effective when used in combination with an aromatase inhibitor or fulvestrant. Side effects may include fatigue, nausea, infections, hair loss, and decreased white blood cell counts, among others. Despite being expensive, most insurance plans cover its cost and manufacturers may offer financial assistance programs. Ibrance may also become a valuable treatment option for other types of cancer in the future.