Diphenhydramine and Lorazepam Co-administration Interval
No specific evidence-based waiting interval is established for taking lorazepam after diphenhydramine. [1] Both diphenhydramine and lorazepam can cause central nervous system (CNS) depression, which can be additive. [2]
Interaction Severity and Clinical Risk
Lorazepam labeling states that benzodiazepines, including lorazepam, produce increased CNS-depressant effects when administered with other CNS depressants, including sedative antihistamines such as diphenhydramine. [2] Lorazepam labeling also states that benzodiazepines used with other CNS depressants can lead to potentially fatal respiratory depression. [2] The FDA also warns that benzodiazepines have serious risks when combined with other CNS depressants. [3]
Practical Timing Guidance
A specific “hours after Benadryl” rule is not provided in regulatory labeling. [2] Safe practice is to avoid taking lorazepam until diphenhydramine dosing has been discussed with the prescriber or until the clinician managing lorazepam determines it is appropriate. [2] If lorazepam is prescribed to be taken despite diphenhydramine exposure, additional caution is required because combined sedation may be excessive or prolonged. [2]
Monitoring for Excess Sedation or Breathing Problems
Sedation and respiratory depression risk increase with concomitant CNS depressants. [2] During combined use, monitoring for excessive or prolonged CNS depression and respiratory depression is recommended. [2]
Safety Actions to Reduce Risk
Operating dangerous machinery or driving should be avoided until the effects of both medications are known. [2] Alcohol and other CNS depressants should be avoided because they further increase CNS-depressant effects. [2]
When Urgent Medical Care Is Needed
Emergency care is indicated for severe drowsiness, inability to stay awake, slowed or difficult breathing, bluish lips or face, or fainting after combined use due to risk of serious respiratory depression. [2]