What is the onset and duration of action of lorazepam in an adult patient with anxiety or insomnia?

Onset and Duration of Lorazepam

Lorazepam begins working within 15-20 minutes when given intravenously or intramuscularly, with peak effects occurring at 30-90 minutes, and clinical effects lasting 6-8 hours. 1

Route-Specific Onset Times

Intravenous Administration

• Onset: 5-10 minutes 1
• Peak effect: 15-30 minutes 1
• Duration: 2 hours at peak, with effects lasting 6-8 hours total 1

Intramuscular Administration

• Onset: 15-20 minutes 1
• Peak effect: 60 minutes 1
• Duration: 6-8 hours 1
• Mean time to sedation in agitated patients: 32.2 minutes 1

Oral Administration

• Onset: 20-30 minutes 1
• Peak effect: 30-90 minutes (most commonly 60-90 minutes) 2, 3
• Duration: 6-8 hours 1
• Peak plasma concentration occurs at approximately 2 hours, which corresponds to maximal clinical effects 4

Clinical Context for Acute Agitation

When used for rapid tranquilization of agitated patients, lorazepam demonstrates slower onset compared to alternatives like midazolam (18.3 minutes) or haloperidol (28.3 minutes), making it less ideal as monotherapy for acute behavioral emergencies 1. However, combination therapy with haloperidol 5 mg + lorazepam 2 mg IM produces faster sedation and superior agitation control compared to either agent alone 5, 6.

Important Pharmacokinetic Considerations

• Elimination half-life: 8-15 hours (approximately 12 hours average) 1, 4
• The relatively long half-life means that drowsiness and sedation persist for at least 4 hours even after IV administration 2
• Delayed emergence from sedation can occur with prolonged administration due to saturation of peripheral tissues, particularly in elderly patients or those with hepatic/renal dysfunction 1

Critical Clinical Pitfalls

• Do not expect immediate sedation: Even with IV administration, there is a 30-40 minute delay before maximum sedative effect occurs 2
• IM injection has significant drawbacks: High frequency of pain at injection site and paradoxical restlessness persisting for 20-40 minutes, which may offset the slightly faster onset compared to oral administration 2
• Propylene glycol toxicity risk: Parenteral formulations contain propylene glycol that can cause metabolic acidosis and acute kidney injury with prolonged use 1
• Respiratory depression risk: Particularly when combined with opioids or in patients with baseline respiratory insufficiency 1