You should seek medical help immediately if you witness any allergic reactions such as swelling face, tongue, lips or throat, hives, difficulty breathing. Just like other narcotic drugs, fentanyl can slow your thinking and breathing. You might die if the breath is too weak. Your doctor will monitor any of the side effects that might clear up within minutes after stopping taking fentanyl or reducing the dose. Other side effects that are common with fentanyl are muscle stiffness, high blood pressure, anxiety, confusion, vision problems, dizziness, fainting, light head, sweating, itching, and pounding or ears and neck. Note that this is not a complete list of side effects and you might witness other side effects. Consult your doctor concerning any of the above side effects.