Short answer: kombucha is sweet tea fermented for one to two weeks by a floating culture (SCOBY) dominated by acetic acid bacteria, so it’s tart, vinegary and contains caffeine. Water kefir is sugar water fermented for one to two days by translucent grains dominated by lactic acid bacteria, so it’s milder, fruitier and caffeine-free. Both contain live microbes and some alcohol, and neither has much human evidence for health benefits. Choose by taste, caffeine and how much time you want to spend brewing.
Side by Side
| Kombucha | Water kefir | |
|---|---|---|
| Base | Sweetened black or green tea | Sugar water, usually with a dried fig or other fruit |
| Culture | SCOBY: a cellulose mat plus the liquid | Water kefir grains: a dextran gel |
| Brewing time | About 7–14 days | About 24–48 hours |
| Main bacteria | Acetic acid bacteria (Gluconacetobacter), some Lactobacillus | Lactic acid bacteria (Lactobacillus), a bifidobacterium |
| Main yeasts | Zygosaccharomyces and others | Saccharomyces cerevisiae, Dekkera |
| Taste | Tart, vinegary, tea notes | Mild, lightly sour, fruity |
| Caffeine | Yes, from the tea | None |
| Alcohol | Can exceed 0.5%, especially when homemade | About 2.6% after 72 hours in one study |
What’s Living in Them
In a sequencing study of kombucha samples, Gluconacetobacter made up more than 85% of the bacteria in most samples, with a sizeable Lactobacillus population (up to 30%) and only traces of Acetobacter; the yeasts in the drink were more than 95% Zygosaccharomyces (Marsh et al., 2014). Acetic acid bacteria give kombucha its vinegary edge. In water kefir, the main microbes were the lactic acid bacteria Lactobacillus casei/paracasei, L. harbinensis and L. hilgardii, a bifidobacterium and the yeasts Saccharomyces cerevisiae and Dekkera bruxellensis (Laureys & De Vuyst, 2014). Both communities vary from culture to culture, so there’s no fixed species count or “billions per glass” figure for either.
Alcohol
Yeasts in both drinks make alcohol. In the US, kombucha at 0.5% alcohol or more is regulated as an alcoholic beverage (TTB), and when New Zealand researchers tested commercial kombucha, 22% of samples exceeded that country’s 1.15% limit (Nordstrom et al., 2026). Water kefir reached about 2.6% alcohol by volume after 72 hours of first fermentation in a Belgian study (Laureys & De Vuyst, 2014), and sealed, sweetened second fermentations of either drink can add more. Neither is a children’s soft drink, and both are worth avoiding in pregnancy or if you don’t drink alcohol.
What the Evidence Shows
Much less than the marketing suggests. A 2019 systematic review of kombucha found only one study reporting results in humans (Kapp & Sumner, 2019); since then, a 12-person pilot trial in people with diabetes reported lower fasting blood sugar after four weeks of kombucha compared with baseline, using drinks donated by the maker (Mendelson et al., 2023). For water kefir, one 14-day study of 40 adults without a control group found shifts in gut bacteria, and about a third of participants reported flatulence and a quarter bloating (de Mel et al., 2026). That’s not evidence for detox, immunity, weight or mood claims for either drink.
Safety
Kombucha is acidic enough to leach metal from glazes: lead poisoning has been reported from kombucha brewed in a ceramic pot (Phan et al., 1998) and in a Spanish family that drank kombucha made and sold in ceramic containers (Munilla García et al., 2023). Brew it in glass. In 1995, the CDC advised caution after two Iowa women fell severely ill (one died) after drinking kombucha from the same home culture; the cause wasn’t established (CDC, 1995). For either drink, discard batches with fuzzy mould or a rotten smell, and use bottles made for carbonated drinks if you seal them.
Which Should You Brew?
- Choose kombucha if you like tart, tea-based drinks, don’t mind caffeine, and are happy to wait a week or two. Start with our kombucha brewing guide and second fermentation flavours.
- Choose water kefir if you want something milder and caffeine-free that’s ready in a day or two. See how to make water kefir and our water kefir soda recipes.