In the 2nd option for analgesia in the TCCC Guidelines, when the casualty is in mild to moderate pain and IS NOT in shock or respiratory distress or at significant risk of developing either condition, fentanyl is the medication of choice.

Contraindications include fentanyl allergy, significant hypotension, MAO inhibitors, myasthenia gravis, potential benefits may warrant use in pregnant women despite potential risks if the alternative is worse. 

If the casualty is in moderate to severe pain, and the casualty is in shock or at risk of shock or pulmonary compromise, ketamine is the agent of choice. 

Fentanyl is an opioid, and all opiates have the potential for causing circulatory and respiratory depression. The potential for opioid analgesics to exacerbate hypoxia and hypotension and therefore cause secondary brain injury in casualties with moderate-to-severe traumatic brain injury (TBI) makes them unsuitable for use in these casualties as well. So, the contraindications for opioids, including fentanyl, are:

Additionally, opioids should be avoided in patients with injuries that are likely to result in hemorrhagic shock (such as poorly controlled junctional hemorrhage or penetrating torso trauma) and in casualties with airway injuries, penetrating chest injuries, severe blunt trauma to the chest, or possible pulmonary blast injury.