What is the bioavailability of ketamine when administered intramuscularly compared to intravenously?

Bioavailability of Ketamine: IM vs IV

The bioavailability of intramuscular (IM) ketamine is approximately 41-93% compared to 100% for intravenous (IV) administration, with the most recent high-quality pediatric study demonstrating 41% bioavailability.

Pharmacokinetic Evidence

The bioavailability difference between routes is substantial and clinically relevant:

• IV ketamine: 100% bioavailability by definition (direct vascular access)
• IM ketamine: 41% bioavailability in the most recent pediatric population study 1, though an older adult study reported 93% 2

The discrepancy between these values likely reflects methodological differences and population variations. The 2018 pediatric study by population pharmacokinetics modeling represents the highest quality evidence, using nonlinear mixed-effects methods in 113 children and is more applicable to contemporary clinical practice 1.

Clinical Implications for Dosing

This bioavailability difference directly explains the dosing protocols observed in clinical practice:

Standard dosing reflects the bioavailability gap:

• IV ketamine: 1-2 mg/kg for procedural sedation 3
• IM ketamine: 4-8 mg/kg for equivalent effect 3, 1

The FDA-approved dosing confirms this pattern 4, 5:

• IV induction: 1-4.5 mg/kg (average 2 mg/kg)
• IM induction: 6.5-13 mg/kg (average 9-13 mg/kg)

The IM dose is approximately 3-4 times higher than IV to compensate for reduced bioavailability.

Onset and Duration Differences

Beyond bioavailability, the routes differ significantly in pharmacodynamics:

Onset time:

• IV: 30-96 seconds 3
• IM: 3-5 minutes 3

Duration of effect:

• IV: 5-10 minutes of surgical anesthesia 4
• IM: 12-25 minutes of surgical anesthesia 5

The McCarty study directly compared both routes and found IV onset averaged 96 seconds versus IM onset of 4 minutes 42 seconds, while recovery times were similar (IV: 84 min vs IM: 90 min) 3.

First-Pass Metabolism Consideration

The lower bioavailability of IM ketamine compared to IV is not due to first-pass hepatic metabolism (which affects oral administration dramatically, reducing bioavailability to only 17% 2). Rather, IM bioavailability reduction reflects:

• Incomplete absorption from muscle tissue
• Variable absorption rates depending on injection site
• Potential local tissue binding

Practical Clinical Guidance

Choose IV when:

• Rapid onset is critical (emergency situations)
• Precise titration is needed
• Vascular access is already established
• Shorter procedures are planned

Choose IM when:

• Vascular access is difficult or unavailable
• Longer duration of effect is desired
• Patient cooperation for IV placement is limited
• Slightly delayed onset is acceptable

The bioavailability difference necessitates careful dose adjustment when switching between routes—never use the same mg/kg dose for both routes. The IM dose must be 3-4 times higher to achieve equivalent plasma concentrations and clinical effect.