Benzodiazepine Equivalence: Lorazepam vs. Midazolam
1 mg of oral lorazepam (PO) is equivalent to approximately 5 mg of intramuscular midazolam (IM), not 3 mg as suggested in the question. 1
Pharmacokinetic Comparison
Lorazepam
- Onset of action: 15-20 minutes (PO), 1-5 minutes (IV)
- Duration: 8-15 hours
- Standard dosing: 0.05-0.1 mg/kg
- Metabolic pathway: Direct glucuronidation without active metabolites
- Considerations: Contains propylene glycol as a diluent in parenteral formulations, which can cause toxicity 2
Midazolam
- Onset of action: 5-15 minutes (IM)
- Duration: 30-60 minutes (shorter than lorazepam)
- Standard dosing: 0.2 mg/kg (IM)
- Metabolic pathway: Hepatic metabolism with active metabolites that may accumulate in renal dysfunction 2
Conversion Factors
The potency relationship between these benzodiazepines is important for clinical practice:
- Lorazepam is approximately 5 times more potent than midazolam when comparing oral lorazepam to intramuscular midazolam 1
- For a 70 kg adult, this translates to 1 mg lorazepam PO ≈ 5 mg midazolam IM 1
Clinical Implications
Sedation Efficacy
- Both medications are effective for sedation, but have different pharmacokinetic profiles
- Midazolam has a faster onset but shorter duration compared to lorazepam
- Lorazepam provides more predictable sedation levels and is easier to manage for long-term sedation 3
Cost Considerations
- Lorazepam therapy can be approximately ten times less expensive than midazolam for long-term sedation 3
Special Populations
- Elderly patients: More sensitive to benzodiazepine effects; require dose reduction of 20% or more 1
- Hepatic/renal impairment: Lorazepam may be preferred due to simpler metabolism with no active metabolites 4
- ICU patients: Emergence times from sedation after a 72-hour infusion are significantly longer with lorazepam (11.9-31.1 hours) compared to midazolam (3.6-14.9 hours) 5
Common Pitfalls
Incorrect conversion ratios: Many institutions and online sources vary in their conversion guidelines; using incorrect ratios can lead to under or oversedation 4
Failure to account for route of administration: The equivalence ratio changes depending on whether comparing oral, IV, or IM routes
Overlooking propylene glycol toxicity: Parenteral lorazepam contains propylene glycol which can cause metabolic acidosis and acute kidney injury at daily IV doses as low as 1 mg/kg 2
Ignoring patient-specific factors: Age, hepatic/renal function, and concomitant medications significantly affect benzodiazepine metabolism and clearance
When converting between these medications, it's safest to start with a lower equivalent dose and titrate as needed based on clinical response, particularly in high-risk populations.