For which of the following is Fentanyl primarily used?

Fentanyl is primarily used for pain management due to its properties as a potent synthetic opioid analgesic. It works by binding to the body's opioid receptors, which are located in the brain and the nervous system, effectively blocking the perception of pain and producing sedation. This makes it particularly valuable in both acute and chronic pain scenarios, including post-surgical pain management and for patients with severe pain who may not respond well to other pain medications.

The other options do not align with Fentanyl's pharmacological use. Allergy relief typically involves antihistamines, anxiety prevention may involve anxiolytics, and antibiotic therapy is reserved for bacterial infections. Each of these areas requires different classes of medications, thus highlighting Fentanyl's specific role in managing pain rather than addressing other medical conditions.