The following severe side effects have not been reported with intranasal fentanyl but have been reported with other forms of fentanyl (oral, patch or intravenous): allergic reactions (rash; hives; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); excessive dizziness; excessive drowsiness; fainting; fatigue; hallucinations; muscle rigidity; seizures; shock (changes in skin color); slow heartbeat; respiratory depression or slowed breathing; trouble breathing and feeling of weakness.