Verapamil can raise the level of fentanyl in your blood. When that happens, fentanyl's side effects can get stronger or last longer. It can also slow your breathing to a dangerous level, which can be fatal. If you take both, your doctor should know so they can watch you closely.
These medicines can affect each other in a serious way. Do not start taking them together unless your doctor has said it is OK. If you already take both, do not stop either one on your own: speak to your doctor or pharmacist soon.