Morphine vs Fentanyl

## Overview

Fentanyl is 80–100× more potent than morphine by weight and has distinct pharmacokinetics due to its extreme lipophilicity. Its rapid onset (seconds IV, minutes transmucosal) and short duration (without patch accumulation) make it suited for different pain scenarios than morphine.

## Mechanism of Action

Both are full mu-opioid receptor agonists. Fentanyl's lipophilicity enables rapid CNS penetration and onset but also rapid redistribution away from the CNS, explaining its short duration of action with IV bolus dosing.

## Pharmacokinetics

Fentanyl: highly lipophilic (logP ~4.0), rapid IV onset (~1–2 min), IV duration 30–60 min with single dose (context-sensitive half-life increases significantly with prolonged infusion). CYP3A4 metabolism to inactive norfentanyl. No active metabolites. Available as: IV, transdermal patch (72-h steady state), buccal/sublingual/intranasal (transmucosal).

Morphine: moderate lipophilicity, IV onset 5–10 min, oral available, active M6G metabolite.

## Renal Considerations

Fentanyl's major advantage in CKD: no active metabolites, making it safer for continuous infusion in renal failure. Morphine's M6G accumulates in CKD, causing prolonged opioid effects. Fentanyl is the preferred opioid in ESRD and dialysis patients for this reason.

## Clinical Applications

Fentanyl IV: procedural sedation, anesthesia. Transdermal fentanyl: chronic cancer pain (requires stable opioid requirement). Buccal/SL fentanyl: breakthrough cancer pain. Morphine: acute severe pain, palliative care. Both are WHO Step 3 opioids.

## Key Takeaways

- Fentanyl: 80–100× morphine potency, no active metabolites, preferred in CKD/ESRD
- Fentanyl: multiple routes (IV, patch, transmucosal); morphine: IV/PO
- Context-sensitive half-life: fentanyl accumulates with prolonged ICU infusion