FENTANYL
Clinical safety rating: avoid
Positive evidence of fetus risks but benefits may outweigh risks in some cases
Fentanyl is a synthetic opioid that primarily acts as a μ-opioid receptor agonist. It binds to μ-opioid receptors in the central nervous system (CNS), leading to G-protein-coupled receptor activation, inhibition of adenylate cyclase, decreased cAMP production, and modulation of ion channels (e.g., increased potassium efflux, decreased calcium influx). This results in hyperpolarization of neurons and reduced neurotransmitter release, producing analgesia, sedation, and euphoria. Fentanyl also has high lipid solubility, allowing rapid CNS penetration and a fast onset of action.
| Metabolism | Fentanyl undergoes extensive hepatic metabolism primarily via CYP3A4 N-dealkylation to norfentanyl (inactive) and other minor metabolites. Approximately 75% of the dose is excreted as metabolites in urine (primarily norfentanyl) and about 9% in feces. Less than 7% is excreted unchanged in urine. The terminal half-life is 3–12 hours, influenced by factors such as hepatic function and age. |
| Excretion | Primarily hepatic metabolism to norfentanyl and other inactive metabolites; renal excretion of metabolites accounts for ~75% of the dose (10% unchanged), with ~9% excreted in feces. |
| Half-life | Terminal elimination half-life is 3–12 hours (mean ~7 hours) in adults; prolonged in elderly, hepatic impairment, or with continuous infusion due to context-sensitive half-life. |
| Protein binding | ~80–85% bound primarily to albumin and alpha-1-acid glycoprotein. |
| Volume of Distribution | Vd: 3–8 L/kg (mean ~4 L/kg), indicating extensive tissue distribution and high lipophilicity. |
| Bioavailability | Transdermal: ~92%; Transmucosal (buccal): ~50%; Oral transmucosal lozenge: ~33%; Intranasal: ~50–70%; Oral (swallowed): very low due to first-pass metabolism (~30% but variable). |
| Onset of Action | IV: 30–60 seconds; IM: 7–15 minutes; Transdermal: 12–24 hours to steady state; Transmucosal (buccal/lozenge): 5–15 minutes; Intranasal: 5–10 minutes. |
| Duration of Action | IV/IM: 30–60 minutes for single dose (analgesia); Transdermal: 72 hours per patch; Transmucosal: 1–2 hours; Context-sensitive half-life prolongs with infusion duration. |
25-100 mcg IV every 1-2 hours as needed; 50-100 mcg IM every 1-2 hours; transdermal patch: 12.5-100 mcg/h every 72 hours; transmucosal: 200-1600 mcg as single dose.
| Dosage form | TABLET |
| Renal impairment | GFR 30-50: use with caution, consider dose reduction by 25-50%; GFR <30: avoid or initiate at 50% of usual dose and titrate slowly; anuric patients: significant accumulation, consider alternative. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50%; Child-Pugh C: avoid or use with extreme caution, reduce dose by 75%. |
| Pediatric use | IV: 1-2 mcg/kg every 2-4 hours; transdermal: not recommended in opioid-naïve children <2 years, start at 12.5 mcg/h if >50 kg; transmucosal: 5-15 mcg/kg as single dose. |
| Geriatric use | Start at 50% of usual adult dose, titrate cautiously by 25% increments; avoid transdermal in opioid-naïve elderly; monitor for respiratory depression and cognitive impairment. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
CNS depressants including alcohol and benzodiazepines increase sedation risk Life-threatening respiratory depression may occur.
| Breastfeeding | Fentanyl is excreted into breast milk. Milk-to-plasma ratio is approximately 0.4. Avoid use in lactating women who are poor metabolizers or receive high doses due to risk of infant sedation and respiratory depression. |
| Teratogenic Risk | First trimester: Limited data; no major malformations reported. Second and third trimesters: Chronic maternal use may lead to neonatal opioid withdrawal syndrome. High doses near term may cause respiratory depression and neonatal abstinence syndrome. |
■ FDA Black Box Warning
WARNING: RISK OF RESPIRATORY DEPRESSION, ADDICTION, ABUSE, AND MISUSE; LIFE-THREATENING RESPIRATORY DEPRESSION; ACCIDENTAL EXPOSURE; NEONATAL OPIOID WITHDRAWAL SYNDROME; INTERACTION WITH ALCOHOL; RISKS FROM CONCOMITANT USE WITH BENZODIAZEPINES OR OTHER CNS DEPRESSANTS; and RISK OF MEDICATION ERRORS (especially with transmucosal formulations).
| Common Effects | Constipation |
| Serious Effects |
["Hypersensitivity to fentanyl or any component of the formulation","Significant respiratory depression (in unmonitored settings or without resuscitative equipment)","Acute or severe bronchial asthma","Paralytic ileus (known or suspected)","Concurrent use of monoamine oxidase inhibitors (MAOIs) or within 14 days of such therapy","Use in opioid-naive patients for transmucosal immediate-release fentanyl (due to risk of fatal respiratory depression)","Acute abdomen (relative contraindication; may obscure diagnosis)"]
| Precautions | ["Life-threatening respiratory depression: risk dose-dependent; monitor respiratory function, especially during initiation and dose escalation.","Addiction, abuse, and misuse: can occur even at recommended doses; screen patients for risk.","Neonatal opioid withdrawal syndrome: prolonged use during pregnancy can result in withdrawal in the newborn.","Interaction with CNS depressants: concomitant use with benzodiazepines or alcohol may cause profound sedation, respiratory depression, coma, and death.","Accidental exposure: especially with transdermal patches; can be fatal.","Risks from use in patients with head injury or increased intracranial pressure: may obscure neurological signs.","Severe hypotension: in patients with compromised blood volume or concomitant use of drugs that depress blood pressure.","Bradycardia and heart block: use with caution in patients with bradyarrhythmias.","Seizures: may exacerbate seizure disorders.","Serotonin syndrome: when used with serotonergic drugs.","Adrenal insufficiency: with prolonged use.","Severe injection site reactions: with injectable formulations.","Risk of medication errors: especially with different formulations (e.g., transdermal vs. transmucosal)."] |
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| Fetal Monitoring |
| Maternal: heart rate, blood pressure, respiratory rate, oxygen saturation, pain/sedation level. Fetal: heart rate monitoring during labor; assess for opioid withdrawal in newborn if chronic use. |
| Fertility Effects | Fentanyl may affect fertility in both males and females. In males, it can reduce libido and cause erectile dysfunction. In females, it may disrupt menstrual cycle and reduce fertility. Use may be associated with hypogonadism. |