Pharmacokinetic Interactions between Valproic Acid and Lorazepam (PIVOtAL Study): A Review of Site-Specific Practices.

BACKGROUND

Coadministration of lorazepam and valproic acid is identified by tertiary references as causing a major drug interaction that requires therapy modification and dosage adjustments. The proposed mechanism involves inhibition of lorazepam glucuronidation via direct inhibition of uridine 5'-diphosphate-glucuronosyltransferase enzymes by valproic acid. However, the clinical significance of this interaction is unclear.

OBJECTIVES

To identify site-specific practices and assess clinical responses to the interaction between valproic acid and lorazepam.

METHODS

A chart review was conducted for patients over 18 years of age who were admitted, from September 2008 to September 2014 inclusive, to the psychiatry or neurology service at Vancouver General Hospital, Vancouver, British Columbia, and who received concomitant valproic acid and lorazepam therapy.

RESULTS

Of the 30 patients included in the chart review, 12 (40%) received an intervention. A total of 8 (27%) patients experienced an adverse drug reaction (ADR), such as drowsiness and dizziness. Seven of these 8 patients were among those who received an intervention. The mean dosage (± standard deviation) of lorazepam was 4.2 ± 1.2 mg per day among patients who experienced an ADR and less than 2 mg per day among those who did not experience an ADR.

CONCLUSIONS

The current recommendation from tertiary drug references is to reduce the dose of lorazepam by 50% when this drug is coadministered with valproic acid. However, this recommendation could not be validated through an analysis of patients exposed to this interaction in the clinical setting or through a review of the literature. Further clinical and pharmacokinetic studies are required to determine whether concurrent treatment with lorazepam and valproic acid should be considered as causing a major drug interaction. Until more data are available, clinicians should remain cognizant of the potential for a drug-drug interaction and should use the lowest effective dose of lorazepam when this drug is administered concomitantly with valproic acid.