Coadministration of valproic acid (250 mg twice daily) and efavirenz (600 mg once daily) had no clinically significant effect on efavirenz pharmacokinetics. Limited data suggest there is no clinically significant effect on valproic acid pharmacokinetics. No dosage adjustment is necessary for efavirenz. Patients should be monitored for seizure control.
Sustiva Summary of Product Characteristics, Bristol-Myers Squibb Pharmaceuticals Ltd, April 2015.

Following the unexpected drug-drug interaction between dolutegravir and valproic acid, plasma concentrations of several antiretrovirals drugs were analyzed when administered in presence and absence of valproic acid. Drug concentrations were obtained from the TDM registry of the University Hospital of Torino. A total of 134 individuals receiving concomitantly valproic acid and antiretroviral drugs were identified (dolutegravir, n=29; raltegravir, n=27; atazanavir, n=24; darunavir, n=13; lopinavir, n=4); etravirine, n=10; rilpivirine, n=7; nevirapine, n=3; efavirenz, n=2; maraviroc, n=17). With the exception of dolutegravir, no significant decrease in plasma exposure was observed for the other antiretrovirals.
Calcagno A, Cusato J, Ferrara M et al. Antiretroviral concentrations in the presence and absence of valproic acid. J Antimicrob Agents 2020; 75:1969-1971.

Coadministration of efavirenz (600 mg once daily) and valproic acid (250 mg twice daily) was studied in 11 HIV+ subjects. There were no significant differences in efavirenz AUC (39.86 vs 39.00 µg/ml.h), Cmin (1.44 vs 1.32 µg/ml) or Cmax (2.43 vs 2.03 µg/ml) when given with valproic acid. There was no apparent effect of efavirenz on valproic acid trough or 8h post dose concentrations.
Effects of valproic acid coadministration on plasma efavirenz and lopinavir concentrations in human immunodeficiency virus-infected adults. DiCenzo R, Peterson D, Cruttenden, et al. Antimicrob Agents Chemother, 2004, 48: 4328-4331.