Kombucha tea is a fermented drink made with tea, sugar, bacteria and yeast. Although it's sometimes referred to as kombucha mushroom tea, kombucha is not a mushroom — it's a colony of bacteria and yeast. Kombucha tea is made by adding the colony to sugar and tea, and allowing the mix to ferment. The resulting liquid contains vinegar, B vitamins and a number of other chemical compounds.
Health benefits attributed to kombucha tea include stimulating the immune system, preventing cancer, and improving digestion and liver function. However, there's no scientific evidence to support these health claims.
There have, however, been reports of adverse effects such as stomach upset, infections and allergic reactions in kombucha tea drinkers. Kombucha tea is often brewed in homes under nonsterile conditions, making contamination likely. If ceramic pots are used for brewing, lead poisoning might be a concern — the acids in the tea may leach lead from the ceramic glaze.
In short, there isn't good evidence that kombucha tea delivers on its health claims. At the same time, several cases of harm have been reported. Therefore, the prudent approach is to avoid kombucha tea until more definitive information is available.