Since fentanyl is so powerful and addictive, dosages should be carefully monitored. Caution should be taken when giving fentanyl to elderly or severely debilitated pets, or those with pre existing respiratory problems. Fentanyl should only be used in pregnant pets when the benefits outweigh the risks.
Fentanyl can cause elevated amylase and lipase. Use with CNS depressants can cause additive sedation and respiratory depression. Use with MAOIs is contraindicated and should never be given. Use with opiate antagonists and partial opiate antagonists are going to diminish the analgesic effects.
Naloxone can be used to treat respiratory depression effects after a fentanyl overdose.