It started with a patient reporting an unexpected side effect. A 35 year old ALK-positive man with lung cancer who was on XALKORI (crizotinib) noted that he had markedly diminished libido lower energy that had been worsening while on treatment, despite the fact that his cancer appeared to be responding well  His doctor checked his testosterone (T) level and noted it was well below the normal range, then referred him to the endocrinology clinic for consideration of testosterone replacement therapy, which he decided to do, and which helped with his symptoms.