Bradycardia is common after fentanyl and blood pressure may also fall. Like all opioids, fentanyl may cause respiratory depression due to a reduction in respiratory rate. Initially there is an increase in depth of respiration, but this becomes more shallow as the amount of opioid increases.

Other side-effects include:

• Nausea and vomiting postoperatively
• Urinary retention
• Constipation and itching

Occasionally chest wall rigidity is seen when fentanyl is used in a large dose as part of induction of anaesthesia.