Reviewed on 10/16/2023

What Is Fentanyl and How Does It Work?

Fentanyl is a prescription medication used to treat the symptoms of Surgery Premedication and for General Anesthesia

Fentanyl is available under the following different brand names: Sublimaze

Dosages of Fentanyl

Adult and Pediatric dosage

Injection solution: Schedule II

  • 0.05mg/mL

Surgery Premedication

  • 50-100 mcg/dose IM or slow IV 30-60 min prior surgery
  • Adjunct to regional anesthesia: 25-100 mcg/dose slow IV over 1-2 min

General Anesthesia 

  • Minor surgical procedures: 0.5-2 mcg/kg/dose IV
  • Major surgery: 2-20 mcg/kg/dose initially: 1-2 mcg/kg/hr maintenance infusion IV; discontinue infusion 30-60 min prior to end of surgery; limit total fentanyl doses to 10-15 mcg/kg for fast tracking and early extubation
  • Adjunct to general anesthesia (rarely used): 2-50 mcg/kg/dose IV

Continuous Sedation/Analgesia

  • Pediatric Dosage
  • 1-12 years: 0.5-2 mcg/kg IV given 3 min prior to procedure; may repeat every 1-2 hours
  • Over 12 years: 0.5-2 mcg/kg/dose; not to exceed 50 mcg/dose; give 3 min prior to procedure; may repeat in 5 min if necessary; if more than two doses needed, may repeat up to 5 times at 25 mcg/dose maximum

Adjunct Anesthesia

  • Younger than 2 years of age: Safety and efficacy not established
  • Older than 2 years of age: 2-3 mcg/kg IV or IM every 1-2 hours as needed 

Dosage Considerations – Should be Given as Follows: 

  • See "Dosages."

What Are Side Effects Associated with Using Fentanyl?

Common side effects of Fentanyl include:

  • headache, 
  • dizziness, 
  • drowsiness, 
  • pale skin, 
  • weakness, 
  • tiredness, 
  • constipation, 
  • nausea, 
  • vomiting, and
  • swelling in your hands and feet

Serious side effects of Fentanyl include:

  • hives, 
  • difficulty breathing, 
  • swelling of your face, lips, tongue, or throat, 
  • slow breathing with long pauses, 
  • blue colored lips, 
  • difficulty to wake up, 
  • slow heart rate, 
  • sighing, 
  • shallow breathing, 
  • breathing that stops during sleep, 
  • severe drowsiness, 
  • lightheadedness
  • confusion, 
  • extreme fear, 
  • unusual thoughts or behavior, 
  • nausea, 
  • vomiting, 
  • loss of appetite, 
  • dizziness, 
  • worsening tiredness or weakness,
  • agitation, 
  • hallucinations, 
  • fever, 
  • sweating, 
  • shivering, 
  • fast heart rate, 
  • muscle stiffness, 
  • twitching
  • loss of coordination, 
  • nausea, 
  • vomiting, and
  • diarrhea 

Rare side effects of Fentanyl include:

  • none 

This is not a complete list of side effects and other serious side effects or health problems may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.

What Other Drugs Interact with Fentanyl?

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them.  Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider or pharmacist first

  • Fentanyl has severe interactions with the following drugs:
    • alvimopan
    • isocarboxazid
    • phenelzine
    • rasagiline
    • safinamide
    • selegiline
    • selegiline transdermal
    • tranylcypromine 
  • Fentanyl has serious interactions with at least 147 other drugs.
  • Fentanyl has moderate interactions with at least 114 other drugs.
  • Fentanyl has minor interactions with no other drugs. 

This information does not contain all possible interactions or adverse effects.  Visit the RxList Drug Interaction Checker for any drugs interactions. Therefore, before using this product, tell your doctor or pharmacist of all the products you use.  Keep a list of all your medications with you, and share this information with your doctor and pharmacist.  Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns.

What Are Warnings and Precautions for Fentanyl?


Effects of drug abuse

  • None

Short-Term Effects

  • See “What are Side Effects Associated with Using Fentanyl?”

Long-Term Effects

  • See “What Are Side Effects Associated with Using Fentanyl?”


  • Caution in acute pancreatitis, Addison disease, benign prostatic hyperplasia, cardiac arrhythmias, central nervous system (CNS) depression, drug abuse or dependence, emotional lability, gallbladder disease, gastrointestinal (GI) disorder, pseudomembranous colitis, GI surgery, head injury, hypothyroidism or untreated myxedema, intracranial hypertension, brain tumor, toxic psychosis, urethral stricture, urinary tract surgery, seizures, acute alcoholism, delirium tremens, shock, cor pulmonale, chronic pulmonary disease, emphysema, hypercapnia, kyphoscoliosis, severe obesity, renal or hepatic impairment, elderly or debilitated patients
  • Cases of serotonin syndrome, a potentially life-threatening condition, reported with concomitant use of serotonergic drugs; this may occur within the recommended dosage range; the onset of symptoms generally occur within several hours to a few days of concomitant use, but may occur later than that; discontinue therapy immediately if serotonin syndrome is suspected
  • Therapy may cause severe hypotension including orthostatic hypotension and syncope in ambulatory patients; there is increased risk in patients whose ability to maintain blood pressure has already been compromised by a reduced blood volume or concurrent administration of certain CNS depressant drugs (e.g., phenothiazines or general anesthetics); monitor patients for signs of hypotension after initiating or titrating dosage; in patients with circulatory shock, therapy may cause vasodilation that can further reduce cardiac output and blood pressure; avoid therapy in patients with circulatory shock
  • In patients who may be susceptible to intracranial effects of CO2 retention (e.g., those with evidence of increased intracranial pressure or brain tumors), therapy may reduce respiratory drive, and resultant CO2 retention can further increase intracranial pressure; monitor such patients for signs of sedation and respiratory depression, particularly when initiating therapy; opioids may obscure clinical course in a patient with a head injury; avoid the use in patients with impaired consciousness or coma
  • Contraindicated in patients with known or suspected gastrointestinal obstruction, including paralytic ileus; may cause spasm of sphincter of Oddi; opioids may cause increases in serum amylase; monitor patients with biliary tract disease, including acute pancreatitis, for worsening symptoms
  • Therapy may increase frequency of seizures in patients with seizure disorders and in other clinical settings associated with seizures; monitor patients for worsened seizure control during therapy
  • Avoid use of mixed agonist/antagonist (e.g., pentazocine, nalbuphine, and butorphanol) or partial agonist (e.g., buprenorphine) analgesics in patients who are receiving a full opioid agonist analgesic; mixed agonist/antagonist and partial agonist analgesics may reduce analgesic effect and/or precipitate withdrawal symptoms; when discontinuing therapy in physically-dependent patient, gradually taper dosage; do not abruptly discontinue therapy in these patients
  • Warn patients not to drive or operate dangerous machinery unless they are tolerant to effects of drug and know how they will react to medication
  • While serious, life-threatening, or fatal respiratory depression can occur at any time during therapy, risk is greatest during initiation of therapy or following dosage increase; monitor patients closely for respiratory depression, especially within first 24 to 72 hours of initiating therapy with and following dosage increases; accidental ingestion of even one dose, especially by children, can result in respiratory depression and death due to overdose of opioid
  • Opioids can cause sleep-related breathing disorders including central sleep apnea (CSA) and sleep-related hypoxemia; opioid use increases risk of CSA in a dose-dependent fashion; in patients who present with CSA, consider decreasing opioid dosage using best practices for opioid taper
  • Concomitant use with a CYP3A4 inhibitor, such as macrolide antibiotics (e.g., erythromycin), azole-antifungal agents (e.g., ketoconazole), and protease inhibitors (e.g., ritonavir), may increase plasma concentrations of fentanyl and prolong opioid adverse reactions, which may cause potentially fatal respiratory depression, particularly when an inhibitor is added after a stable dose of fentanyl injection is achieved; similarly, discontinuation of a CYP3A4 inducer, such as rifampin, carbamazepine, and phenytoin, in fentanyl-injection treated patients may increase fentanyl plasma concentrations and prolong opioid adverse reactions; when using fentanyl Injection with CYP3A4 inhibitors or discontinuing CYP3A4 inducers in fentanyl-Injection treated patients, monitor patients closely at frequent intervals and consider dosage reduction of fentanyl injection until stable drug effects are achieved
  • Concomitant use of fentanyl injection with CYP3A4 inducers or discontinuation of a CYP3A4 inhibitor could decrease fentanyl plasma concentrations, decrease opioid efficacy or, possibly, lead to a withdrawal syndrome in a patient who had developed physical dependence to fentanyl; when using fentanyl injection with CYP3A4 inducers or discontinuing CYP3A4 inhibitors, monitor patients closely at frequent intervals and consider increasing opioid dosage if needed to maintain adequate analgesia or if symptoms of opioid withdrawal occur
  • Concomitant use of opioids with benzodiazepines or other central nervous system (CNS) depressants, including alcohol, may result in profound sedation, respiratory depression, coma, and death; reserve concomitant prescribing for use in patients for whom alternative treatment options are inadequate; limit dosages and durations to minimum required; follow patients for signs and symptoms of respiratory depression and sedation
  • Profound sedation, respiratory depression, coma, and death may result from concomitant administration with benzodiazepines or other CNS depressants (e.g., non-benzodiazepine sedatives/hypnotics, anxiolytics, tranquilizers, muscle relaxants, general anesthetics, antipsychotics, other opioids, alcohol); because of these risks, reserve concomitant prescribing of these drugs for use in patients for whom alternative treatment options are inadequate
  • Use in patients with acute or severe bronchial asthma in an unmonitored setting or in absence of resuscitative equipment is contraindicated; patients with significant chronic obstructive pulmonary disease or cor pulmonale, and with substantially decreased respiratory reserve, hypoxia, hypercapnia, or pre-existing respiratory depression are at increased risk of decreased respiratory drive including apnea, even at recommended dosages
  • Life-threatening respiratory depression is more likely to occur in elderly, cachectic, or debilitated patients because they may have altered pharmacokinetics or altered clearance compared to younger, healthier patients; monitor closely
  • Monoamine oxidase inhibitors (MAOIs) may potentiate effects of opioid, opioid’s active metabolite, including respiratory depression, coma, and confusion; therapy should not be administered within 14 days of initiating or stopping MAOIs
  • Cases of adrenal insufficiency reported with opioid use, more often following greater than one month of use; symptoms may include nausea, vomiting, anorexia, fatigue, weakness, dizziness, and low blood pressure; if adrenal insufficiency is diagnosed, treat with physiologic replacement doses of corticosteroids; wean patient off of opioid to allow adrenal function to recover and continue corticosteroid treatment until adrenal function recovers; other opioids may be tried as some cases reported use of a different opioid without recurrence of adrenal insufficiency
  • Use caution when selecting dosage for an elderly patient, usually starting at low end of dosing range, reflecting greater frequency of decreased hepatic, renal, or cardiac function and of concomitant disease or other drug therapy; because elderly patients are more likely to have decreased renal function, care should be taken in dose selection, and may be useful to monitor renal function
  • Opioid pharmacokinetics may be altered in patients with renal failure; clearance may be decreased and metabolites may accumulate much higher plasma levels in patients with renal failure as compared to patients with normal renal function; start with a lower than normal dosage or with longer dosing intervals and titrate slowly while monitoring for signs of respiratory depression, sedation, and hypotension
  • Risks of potentially fatal respiratory depression, pruritus (despite little histamine release), and abuse or addiction
  • May produce bradycardia, which may be treated with atropine
  • Patient access to naloxone for emergency treatment of opioid overdose
  • Assess potential need for naloxone; consider prescribing for emergency treatment of opioid overdose
  • Consult on availability and ways to obtain naloxone as permitted by individual state naloxone dispensing and prescribing requirements or guidelines
  • Educate patients regarding the signs and symptoms of respiratory depression and to call 911 or seek immediate emergency medical help in the event of a known or suspected overdose

Pregnancy and Lactation

  • Prolonged use of opioid analgesics during pregnancy for medical or nonmedical purposes can result in physical dependence in the neonate and neonatal opioid withdrawal syndrome shortly after birth; observe newborns for symptoms of neonatal opioid withdrawal syndrome and manage accordingly; opioids cross placenta and may produce respiratory depression and psycho-physiologic effects in neonates; an opioid antagonist, such as naloxone, must be available for reversal of opioid-induced respiratory depression in the neonate; opioid sulfate is not recommended for use in pregnant women during or immediately prior to labor, when other analgesic techniques are more appropriate; opioid analgesics can prolong labor through actions which temporarily reduce strength, duration, and frequency of uterine contractions
    • Fertility
    • Due to effects of androgen deficiency, chronic use of opioids may cause reduced fertility in females and males of reproductive potential; it is not known whether effects on fertility are reversible
  • Opioid is secreted into human milk; in women with normal opioid metabolism (normal CYP2D6 activity), the amount of opioid secreted into human milk is low and dose-dependent; some women are ultra-rapid metabolizers of opioid; these women achieve higher-than-expected serum levels of opioid's active metabolite, opioid, leading to higher-than-expected levels of opioid in breast milk and potentially dangerously high serum opioid levels in their breastfed infants that can potentially lead to serious adverse reactions, including death, in nursing infants
  • Developmental and health benefits of breastfeeding should be considered along with mother’s clinical need for therapy and any potential adverse effects on breastfed infant from therapy or from underlying maternal condition


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