This information is for people who have anxiety disorder. It tells you about kava, a treatment sometimes used for anxiety disorder.
Does it work?
Probably. If you have anxiety disorder, kava may improve your symptoms. But it can damage your liver, so it has been banned from sale in the UK.
What is it?
Kava is an herbal medicine that comes from a pepper plant. It's also known as kava-kava. The scientific name for the plant is Piper methysticum. Many people living in the South Pacific take kava to help them relax and sleep.
How can it help?
Kava can make you feel better in the short term.  One study has found that kava seems to work as well as the antidepressantopipramol and the drug buspirone, which is also used to treat anxiety.  Opipramol is not available in the UK.
How does it work?
Kava contains chemicals called lactones (also known as kavalactones or kavapyrones). These chemicals help you relax. We don't know exactly how lactones make you feel more relaxed. A chemical in your brain called gamma-aminobutyric acid (GABA) slows down your brain by stopping some cells communicating with other cells. Kava might help GABA do this job, making your brain work more slowly and making you feel calmer.
Can it be harmful?
Kava can seriously damage your liver. If your liver is damaged you can get many different symptoms, including pain in your abdomen and yellow skin. If you take a high dose of kava for a long time, you may also lose some hair. 
Kava was banned from sale in the UK in January 2003. Liver damage is probably rare, but we still don't know exactly how many people who take kava will get it. One study reports nine cases of liver damage out of 4,049 people taking kava. 
There doesn't seem to be a safe dose of kava. We don't know why kava damages the liver of some people and not others.
How good is the research on kava?
There is some evidence that kava helps to relieve anxiety. One summary of the research (called a systematic review), which combined the results of three studies, found that found that kava worked better than a dummy treatment (a placebo) at reducing anxiety.  But another study found that kava made no difference to how anxious people felt. 
Another study compared kava with the antidepressant opipramol and the drug buspirone.  It found that kava worked just as well as these two other drugs. Opipramol is not available in the UK.
A placebo is a 'pretend' or dummy treatment that contains no active substances. A placebo is often given to half the people taking part in medical research trials, for comparison with the 'real' treatment. It is made to look and taste identical to the drug treatment being tested, so that people in the studies do not know if they are getting the placebo or the 'real' treatment. Researchers often talk about the 'placebo effect'. This is where patients feel better after having a placebo treatment because they expect to feel better. Tests may indicate that they actually are better. In the same way, people can also get side effects after having a placebo treatment. Drug treatments can also have a 'placebo effect'. This is why, to get a true picture of how well a drug works, it is important to compare it against a placebo treatment.
A systematic review is a thorough look through published research on a particular topic. Only studies that have been carried out to a high standard are included. A systematic review may or may not include a meta-analysis, which is when the results from individual studies are put together.
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