Alfentanil
Alfentanil Hydrochloride
Also known as: Fanaxal · Fentalim · Limifen · Rapifen · alfentanyl · Alfentanilum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceAnalgesia (extra-label) — Intraoperative analgesia in patients with intracranial disease
Verified clinician required
NA
Governing clinical context (3)
- NOTE: In obese patients, calculate doses based on lean body weight.
- Analgesia (extra-label):
- Alfentanil injection should be stored protected from light at controlled room temperature (20°C-25°C; 68°F-77°F).
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Alfentanil is a potent, short-acting synthetic opioid belonging to the phenylpiperidine class, structurally related to fentanyl.
Key characteristics include:
- Rapid Onset & Short Duration: Highly lipophilic, allowing rapid penetration of the blood-brain barrier, making it ideal for short procedures or as a continuous rate infusion (CRI).
- Anesthetic Adjunct: Primarily used to provide profound analgesia and reduce the required dose of induction or maintenance anesthetic agents (e.g., propofol, isoflurane).
- Species Specificity: While veterinary experience is somewhat limited compared to fentanyl, it is noted to be particularly useful in cats as an adjunctive therapy during anesthesia.
- Regulatory Status: It is a Class-II controlled substance due to its high potential for abuse.
Clinical Pearl: Alfentanil is less lipid-soluble than fentanyl but more so than morphine. Its rapid redistribution and clearance make it highly titratable in a critical care or surgical setting.
Mechanism of action
Alfentanil acts primarily as a potent and highly selective agonist at mu (μ) opioid receptors in the central nervous system.
- Mechanism: Binds to pre- and post-synaptic μ-receptors → inhibits adenylate cyclase → decreases intracellular cAMP → promotes opening of inward-rectifying potassium channels → hyperpolarization of the neuron.
- Neurotransmitter Inhibition: Concurrently inhibits voltage-gated calcium channels → decreases the release of excitatory nociceptive neurotransmitters (such as substance P and glutamate) → profound analgesia and sedation.
- Systemic Effects: Causes dose-dependent respiratory depression by decreasing the responsiveness of the brainstem respiratory centers to carbon dioxide.
Safety & warnings
Contraindications
- Patients hypersensitive to opioids
Adverse effects
- Dose-related respiratory depression
- Bradycardia
- CNS depression
- Dose-related skeletal muscle rigidity
- Asystole (rare)
- Hypercarbia (rare)
- Hypersensitivity reactions (rare)
Precautions
Critical Warning: Because of the drug's potency and potential for significant adverse effects, it should only be used in situations where patient vital signs can be continuously monitored.
- Dose Adjustments: Initial dosage reduction may be required in geriatric or debilitated patients, particularly those with diminished cardiopulmonary function or liver disease.
- Muscle Rigidity: Dose-related skeletal muscle rigidity is not uncommon; neuromuscular blockers are routinely used in conjunction.
- Laboratory Artifacts: May increase plasma levels of amylase or lipase secondary to increased biliary tract pressure. Values may be unreliable for 24 hours post-administration.
Drug interactions
May increase the half-life and decrease the clearance of alfentanil leading to prolonged effect and an increased risk of respiratory depression
May produce bradycardia or hypotension if used concurrently with alfentanil
May produce bradycardia or hypotension; respiratory or CNS depression may be exacerbated
Reduces the dose requirements of concurrently administered anaesthetics by at least 50%
Risk of serotonin toxicity
Monitoring
- Anesthetic and/or analgesic efficacy
- Cardiac rate and rhythm
- Respiratory rate and depth
- Pulse oximetry or other methods to measure blood oxygenation
Pharmacokinetics
Half-life
Absorption
In humans, onset of anesthetic action occurs within 2 minutes after IV dosing, and within 5 minutes of IM injection. Peak effects occur approximately 15 minutes after IM injection.
Distribution
Steady state volume of distribution in dogs is about 0.56 L/kg. In humans, Vd is 0.4-1 L/kg. About 90% of the drug is bound to plasma proteins.
Metabolism
Primarily metabolized in the liver to inactive metabolites.
Elimination
Metabolites are excreted by the kidneys into the urine; only about 1% of the drug is excreted unchanged into the urine. Clearance in dogs is approximately 30 mL/kg/minute.
Overdose
Signs of Toxicity
- Severe IV Overdose: Circulatory collapse, pulmonary edema, seizures, cardiac arrest, and death.
- Less Severe Overdose: CNS and respiratory depression, coma, hypotension, muscle flaccidity, and miosis.
Treatment
- Supportive Care: Provide cardiovascular and respiratory support (e.g., mechanical ventilation, IV fluids) as necessary.
- Antidote: Administration of an opiate antagonist such as naloxone. Although alfentanil has a relatively rapid half-life, multiple doses of naloxone may be necessary.
Measurement Precaution: Because of the drug's extreme potency, the use of a tuberculin syringe to measure dosages less than 1 mL, along with a dosage calculation and measurement double-check system, is strongly recommended.
Available products
Formulations
- Injection: 500 micrograms/mL
Human-labeled
- Alfentanil HCl for injection: 500 micrograms (as base)/mL in 2, 5 mL, 10 mL, & 20 mL amps; preservative-free; Alfenta® (Akorn); generic (Abbott); (Rx, C-II)
Regulatory status
Controlled Drug
POM CD SCHEDULE 2
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Alfentanil injection should be stored protected from light at room temperature. Compatible with Normal Saline, D5 in Normal Saline, D5W, and Lactated Ringers in concentrations up to 80 micrograms/mL.
Client information
Alfentanil is a highly potent opioid medication used strictly for anesthesia and severe pain management.
- Professional Use Only: This drug is only administered by veterinary professionals in a clinical setting where advanced patient monitoring (like ECG and oxygen tracking) is available.
- Safety: Because it can slow breathing and heart rate, your pet will be closely monitored throughout the entire procedure.
- Post-Anesthesia: Your pet may be groggy or sleepy after the procedure, which is a normal effect of opioid anesthetics.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.