Side Effects Caverject may cause mild pain, irritation or slight bleeding at the injection site. There is also a possibility to develop one or more of the following side effects: tenderness or swelling of the penis, redness/lumps/unusual curving of the penis, and/or priapism. Long-term use carries a 3% risk of developing fibrosis of the penis. Controlled-dose titration within the context of a medical professional's office is essential to the safety of using the product. cancer is also a possible longterm side effect.