Countless health claims swirl around kombucha, a fermented drink made with tea, sugar, bacteria and yeast.
Although there is little research backing up the would-be superfood benefits of kombucha, some studies have suggested it can help with gut health in place of probiotic supplements, and also support the body's immune system.
There risks of this drink are even clearer, however, particularly for vulnerable groups such as pregnant women, young children and anyone with a weak immune system.
What's more, if it's made at home in unsafe conditions, unwanted bacteria that grows in it can cause allergic reactions, infections and an upset stomach.
In many countries there are also unclear rules defining how the drink is made, and depending on the supplier, kombucha can therefore taste different and have different ingredients.
Who can drink kombucha? Certain groups of people should be careful or avoid it altogether. Because kombucha contains small amounts of alcohol, pregnant women, breastfeeding mothers, infants, small children and dry alcoholics should avoid it.
Germany's Federal Nutrition Centre (BZfE) also warns people with a weak immune system or pre-existing conditions such as kidney failure to steer clear of it out of precaution, especially with unfamiliar products where production methods are not standardized.
Kombucha can also be made at home, but hygiene is a top priority, and it's essential to use sterile containers.
It's also best to use a glass container because the acidity of the fermented drink can dissolve the lead from ceramics with a lead-containing glaze and, in the worst case, lead to poisoning if consumed over a long period of time.
The basic ingredients are usually black or green tea, sugar and SCOBY - a culture of bacteria and yeast. The abbreviation stands for Symbiotic Culture of Bacteria and Yeasts.
First, boil water and leave the sweetened tea to infuse for around 10 minutes. If you like, you can also let it infuse with some fruit, herbs or even vegetables.
Then to the cooled, filtered liquid, add at least 10% of your starter liquid. This helps to ferment the tea at room temperature, which is converted into carbon dioxide and alcohol and further metabolized by the bacteria.
This produces organic acids such as acetic, lactic and gluconic acid. Detailed recipes can be found online and in books. Depending on the mixture and how long it's left to ferment, the finished drink tastes somewhere between sweet and tart or sour.
Important: If mould forms or the colour or smell changes, the entire culture must be thrown away, since consuming kombucha made with this could mean negative health effects.
