Some of the side effects of fentanyl use are constipation, nausea, vomiting, abdominal cramps, sweating, insomnia, fatigue, drowsiness, feeling cold, headache, mood swings, changes in appetite, depression, anxiety, confusion, hallucinations, muscle spasms, tremors, and slowed heart reaction.

https://recoverypartnernetwork.com/drug/opioid/fentanyl-abuse