The review, published in The Cochrane Database of Systematic Reviews (2006, issue 3) identified 22 potentially relevant randomised clinical trials (RCTs), 12 of which met the inclusion criteria.
Seven of these suggest that kava could have a favourable effect on anxiety, when ranked using the Hamilton Anxiety Scale.
"These data imply that, compared with the placebo, kava extract might be an effective symptomatic treatment for anxiety, although, at present, the size of the effect seems to be small," wrote authors Dr M H Pittler and Professor Edzard Ernst of the University of Exeter, UK.
They stressed that the findings are limited to short term efficacy, of one to 24 weeks' use.
Kava is a herb from the pepper family with a long history of use in the Pacific Islands, and more recently in Europe, the US and Australia as a herbal medicine and in foods such as tea, cereal products, smoothies and spirit drinks.
It has been making headlines in the last month since the UK regulatory authorities announced that they are upholding a ban on the herb's use in food and supplement products on account of the safety data, which they say indicate there is some risk of liver toxicity.
Safety has been closely monitored in a number of countries: UK, France, Ireland, Germany and Portugal have withdrawn kava kava from sale, to varying degrees. Canada and Australia advise against consumption while a risk assessment takes place, and the US and New Zealand have also initiated safety assessments, although products remain on sale for the meantime.
But outright bans have proved controversial with herbal groups, who argue that it has a better safety record than many prescription drugs.
When a wave of bans was implemented in 2001-2 the kava export industry in the South Pacific was all but wiped out, plunging many smallholders in island economies like Fiji and Vanuatu, which relied heavily on the herb, into poverty.
The reviewers said that few adverse events were reported in the studies examined, and those there were "were all mild, transient and infrequent".
However they said: "Rigorous trails with large sample sizes are needed to clarify the existing uncertainties. Particularly long-term safety studies of kava are needed."
In 2002, when the UK ban was imposed, Prof Edzard Ernst expressed the opinion that it went too far. "Kava is proven to be effective in treating anxiety and, looking at the total risk, it is safer than synthetic drugs. If we are going to ban kava today, then we should have banned Valium twenty years ago," he said.
Prof Ernst and Dr Pittler are not the only ones calling for long-term safety studies.
Last month Dr Matthias Schmidt, a consultant on herbal drug safety and member of the International Kava Executive Council, told NutraIngredients.com that The only way to convince European governments on the safety and efficacy of kava kava and to restore the industry in the South Pacific is to conduct clinical studies and look further into the mechanism.
Dr Schmidt is currently preparing a study plan for a clinical trial, although University partners and funding are yet to be confirmed. Although this trail is aimed at fully restoring the German market, he said: "This will send a tremendous signal to other countries".