The phosphatidylinositol 3-kinase (PI3K) pathway has attracted immense interest as a therapeutic target for cancer treatment. Idelalisib was the first PI3K inhibitor approved by the US Food and Drug Administration and is utilized in the treatment of relapsed/refractory chronic lymphocytic leukemia/small lymphocytic lymphoma and follicular lymphoma. Copanlisib has subsequently been approved for relapsed follicular lymphoma in patients who have received at least two prior systemic therapies. There are multiple other PI3K agents currently in development; these target various combinations of PI3K isoforms. Despite the therapeutic benefit, there have been concerns about the severe and sometimes fatal adverse effects of this class of drug. Several side effects are unusual and have poorly understood mechanisms; these include autoimmune dysfunction, opportunistic infections, skin toxicity, hypertension, and hyperglycemia. An understanding of these unusual toxicities, as well as a good grasp of management principles, will be important as more PI3K inhibitors are approved and become incorporated into routine practice.