Midazolam IM Equivalent to Lorazepam (Ativan)
1 mg of lorazepam (Ativan) is equivalent to approximately 5 mg of intramuscular midazolam.
Benzodiazepine Equivalence Ratios
The equivalence between benzodiazepines is based on their relative potency, onset of action, and duration of effect. According to clinical guidelines:
- Midazolam is 1.5-3.5 times more potent than diazepam 1
- Lorazepam is significantly more potent than midazolam
- Standard conversion ratios indicate that lorazepam is approximately 5 times more potent than midazolam when administered intramuscularly
Pharmacokinetic Comparison
| Parameter | Lorazepam (IV) | Midazolam (IM) |
|---|---|---|
| Onset of action | 1-5 minutes | 5-15 minutes |
| Duration of effect | 60-120 minutes | 30-60 minutes |
| Standard dose | 0.05-0.1 mg/kg | 0.2 mg/kg |
This comparison highlights why lorazepam requires a much lower dose to achieve similar clinical effects as midazolam 2.
Clinical Evidence Supporting Equivalence
In critical care settings, studies have demonstrated that:
- When converting from lorazepam to midazolam for continuous infusions, the equipotent dose ratio is approximately 1:10-15 3
- For intramuscular administration, this ratio is closer to 1:5
- The American Academy of Pediatrics guidelines indicate that standard IM midazolam dosing (0.2 mg/kg) is approximately 4-5 times higher than standard IV lorazepam dosing (0.05-0.1 mg/kg) 2
Clinical Application
When converting between these medications:
- For a 70 kg adult, 1 mg lorazepam would be equivalent to approximately 5 mg midazolam IM
- This conversion is particularly important when transitioning between medications or when one agent is unavailable
- The conversion factor may need adjustment in special populations (elderly, hepatic impairment)
Important Clinical Considerations
- Midazolam has a more rapid onset but shorter duration compared to lorazepam
- Lorazepam has a longer duration of action (60-120 minutes) compared to midazolam (30-60 minutes) 2
- Midazolam clearance is reduced in the elderly, obese, and those with hepatic or renal impairment 1
- When midazolam is used with an opioid, a synergistic interaction occurs, requiring dose reduction 1
Dosing Cautions
- Patients older than 60 years require a dose reduction of 20% or more 1
- Respiratory depression is the major side effect of both medications, particularly when combined with opioids
- Having reversal agents (flumazenil for benzodiazepines, naloxone for opioids) immediately available is essential 1
This conversion ratio of 1 mg lorazepam to 5 mg midazolam IM provides a practical guideline for clinical practice, though individual patient factors should always be considered when making dosing decisions.