1 Anxiety. 2  The majority of evidence shows that certain kava extracts (extracts standardized to 70% kavalactones) can lower anxiety and might work as well as prescription anti-anxiety medications called low-dose benzodiazepines.3  But it might take up to 8 weeks of treatment to see improvement.regnancy and breast-feeding: Don’t use kava if you are pregnant or breast-feeding. Kava is POSSIBLY UNSAFE when taken by mouth. There is a concern that it might affect the uterus. Also, some of the dangerous chemicals in kava can pass into breast milk and might hurt a breast-fed infant.