Hydrogen Studies is funded by Echo Technologies LLC, which sells hydrogen water products. Echo doesn't decide which studies we cover or how we describe them — see our editorial policy.
Short answer: The negative side effects reported in human trials of hydrogen water have been uncommon and mild: loose stools, more frequent bowel movements, heartburn, headache, and more trips to the bathroom from drinking extra water. None of the trials we reviewed reported a serious adverse event attributed to hydrogen water. But those trials were small (roughly 14 to 60 people) and short (a single session up to 48 weeks), and many did not track side effects carefully. The honest summary is "low risk so far, long-term safety unknown."
What side effects have human studies reported?
Reported complaints are mostly digestive and urinary, and mild. One placebo-controlled trial did find more adverse events with hydrogen water than with placebo.
Here are ten human trials of drinking hydrogen-rich water, with doses exactly as reported. Open-label studies had no comparison group.
| Study | Who took part | Dose (as reported) | Duration | Adverse events reported |
|---|---|---|---|---|
| Nakao 2010 [1], open-label | 20 adults with features of metabolic syndrome | 1.5–2 L/day; 0.55–0.65 mM (magnesium stick) | 8 weeks | 28 events in 13 people; 6 events in 4 people judged possibly related: loose stools (3), more frequent bowel movements (1), heartburn (1), headache (1). All mild, none serious |
| LeBaron 2020 [2], placebo-controlled | 60 adults with metabolic syndrome | >5.5 mmol H₂/day (one tablet in 250 mL, 3×/day) | 24 weeks | All finished; "well tolerated" with no ill effects reported |
| Kura 2022 [3], placebo-controlled | 30 adults with fatty liver disease (NAFLD) | 330 mL 3×/day at >4 mg/L (magnesium tablet) | 8 weeks | All finished "without adverse events" |
| Ishibashi 2012 [4], open-label | 20 adults with rheumatoid arthritis | 530 mL/day at 4–5 ppm | 4 weeks, 4-week break, 4 weeks | Passing urine 1–2 more times than usual (from the extra water); no other adverse effects |
| Ito 2011 [5], open-label, then placebo-controlled crossover | 14, then 22 adults with muscle diseases | 1.0 L/day, then 0.5 L/day; about 0.5 ppm | 12 weeks; 8 weeks | More frequent urination; one mild "floating sensation"; one person's diarrhea worse at first; low blood sugar in one insulin user, which stopped after the insulin dose was lowered |
| Todorovic 2025 [6], placebo-controlled | 36 adults with obesity (35 analyzed) | 1.0 L/day (15 mg H₂/day) | 8 weeks | No severe adverse effects; one person reported more frequent bowel movements |
| Zhang 2025 [7], placebo-controlled | 60 adults with chronic mountain sickness (59 analyzed) | 990 mL/day | 8 weeks | Adverse events in 15 of 30 on hydrogen water vs 11 of 29 on placebo; dizziness most common |
| Yoritaka 2013 [8], placebo-controlled | 17 adults with Parkinson's disease (analyzed) | 1,000 mL/day | 48 weeks | Described as "safe and well tolerated"; details not given in the abstract |
| Sim 2020 [9], placebo-controlled | 41 healthy adults (38 finished) | 1.5 L/day at 0.753 mg/L | 4 weeks | None described; 2 hydrogen-group dropouts (days 4 and 10), reasons not stated |
| Valenta 2022 [10], placebo-controlled crossover | 24 trained male runners (finished) | 1,260 mL over the 2 hours before exercise; 0.9 ppm | One session per condition | Not reported; 4 didn't finish due to "medical complications or technical problems" (not described) |
A few details matter more than the table can show:
- Most symptom reports came from studies without a control group. In the Nakao study, 13 of 20 people had some adverse event over 8 weeks, but investigators judged only 6 events possibly related to the water [1]. Without a placebo group, there's no way to know how many would have happened anyway.
- A trial that asked directly about gut symptoms found little. The 2025 obesity trial asked about stomach upset, bloating, constipation, diarrhea, nausea and vomiting. One person in the hydrogen group reported more frequent bowel movements [6].
- One trial leaned the other way. In the chronic mountain sickness trial, 50% of the hydrogen group had an adverse event versus 37% on placebo, mostly dizziness [7]. The abstract doesn't say the gap was statistically significant, and with 59 people it may be chance. It also saw a rise in malondialdehyde, a fat-oxidation marker, as did a fatty-liver trial [3]. The mountain-sickness authors concluded that hydrogen water didn't reduce oxidative damage and might increase oxidative risk in people with a higher BMI [7]; what that means for health is unknown.
Why is hydrogen considered low-toxicity?
Your body already makes and clears hydrogen, a glass of hydrogen water contains only milligrams of it, and people have breathed far larger amounts in specialized deep-sea diving. Lab, animal and regulatory evidence point the same way, though none of it proves long-term safety in people.
- You already carry it. Gut bacteria produce large amounts of hydrogen [11], and eating fiber increases that production [2]. In one study, three volunteers' breath held about 14–16 ppm hydrogen before they drank anything. After high-hydrogen water, breath levels peaked within 5 minutes and were back near baseline within about an hour [4].
- The dose is small. Ordinary hydrogen-saturated water holds about 1.6 ppm. The pressurized water in one trial held 4–5 ppm, or 2.1–2.7 mg of hydrogen per 530 mL bottle [4]. One trial gave 15 mg a day [6]. Our explainer on what hydrogen water is covers ppm in more detail.
- Divers have breathed it under pressure. Hydreliox, a hydrogen–helium–oxygen breathing mix, was used in record dives: six commercial and French Navy divers worked at 534 m of seawater in 1988, and an onshore record dive to 701 m followed in 1992 [12]. In a 500 m open-sea dive with a mix containing 49% hydrogen, researchers detected a narcotic effect from the hydrogen, though the divers completed their work [13]. That's an extreme exposure, very different from drinking a few milligrams.
- Lab and animal tests. Water from one electrolysis design showed no signs of causing mutations or chromosome damage in lab tests, and no toxic changes in rats given it for 28 days [14]. That's lab and animal evidence only.
- Regulatory status. In 2014 the US FDA answered "no questions" to a GRAS ("generally recognized as safe") notice for hydrogen gas as an ingredient in drinking water, flavored beverages and sodas at up to 2.14% by volume (GRN 520) [15]. That means FDA didn't object to the company's own safety conclusion. It is not an FDA approval, and it says nothing about health benefits.
Does the delivery method change the risk?
Yes. The molecule is the same, but each delivery method brings its own practical risks.
Drinking water (pre-filled cans or pouches, gas-infused water). The issues are those in the table: mild digestive effects and extra fluid.
Inhaled gas. Hydrogen is flammable in air at concentrations from about 4% to 75%, and at high concentrations it can displace the oxygen in the air [16]. In a hospital safety study, 8 healthy adults breathed 2.4% hydrogen in air for 24 to 72 hours with no clinically significant adverse events [17]. See hydrogen inhalation side effects for more.
Magnesium tablets and sticks. These make hydrogen by reacting magnesium metal with water (Mg + 2H₂O → Mg(OH)₂ + H₂) [1], sometimes with an acid added [3]. A tablet dissolves into the drink, so you swallow its magnesium along with the hydrogen [3]. By the chemistry, each milligram of hydrogen made this way uses about 12 mg of magnesium. For adults, the upper limit for magnesium from supplements is 350 mg a day, a limit set because of diarrhea [18, 19]. Magnesium toxicity is more likely when the kidneys can't clear it [18], so check the label. Sticks work differently: one study measured less than 1.0 mg/L of magnesium in stick-treated water [1]. More in H2 tablet side effects.
Electrolysis devices. These split water with electricity, and designs differ. The rat and cell tests above covered one design, which used a cell without a membrane plus an activated-carbon filter [14], and the results apply only to that design. We found no peer-reviewed human studies comparing electrode materials or designs in consumer devices, so we can't say which are safer. If you use one, look for independent lab results on the water it produces.
Who should be cautious or talk to a clinician first?
Check with a clinician first if you're pregnant or breastfeeding, have kidney disease or are on dialysis, are giving it to a child, or take insulin or other glucose-lowering drugs. That's mostly because these groups haven't been studied, not because harm has been shown.
- Pregnancy and breastfeeding. We found no human trials. Trials have excluded these groups: one ruled out pregnant and breastfeeding women [1], another pregnancy and contraceptive use [2]. The pregnancy research we found is in animals, such as pregnant mice [20].
- Kidney disease. Magnesium from tablets is the main concern, because failing kidneys can't remove excess magnesium well [18]. At least one trial excluded people with kidney disease [1]. Our article on hydrogen water and kidneys covers that research.
- Dialysis. People on hemodialysis are usually told to limit liquids between treatments [21], and hydrogen water counts toward that. Adding hydrogen to the dialysis solution itself has been studied in dialysis patients, which is a different, supervised situation (see our kidney article).
- Children. We found no trials of children drinking hydrogen water. Supplemental magnesium limits are lower for children, from 65 to 350 mg a day depending on age [18].
- Medications. Drug interactions have not been formally studied. The one signal we found was a single low-blood-sugar case in an insulin user [5]. One case can't show cause, but if you use insulin, monitor your levels and tell your clinician.
How much hydrogen water is too much?
No upper limit has been set for hydrogen in drinking water. In practice, the limits are how much water you can comfortably drink and, with tablets, how much magnesium you take in.
Trials have used 0.5 L a day [5] up to 1.5–2 L a day [1], and one sports study had swimmers drink 2,520 mL on a test day [23]. See our guide to how much hydrogen water per day.
Volume is the practical barrier. Researchers found that 1.0 L a day was "not readily accommodated" by most of their patients, especially in winter [5]. In the arthritis trial, people urinated more often, and one person stopped drinking it daily during winter and left the study [4]. Bloating wasn't reported in the trial that asked about it [6], but loose stools were the most common possibly-related effect in the open-label study [1]. With tablets, add up the magnesium per tablet against the 350 mg daily limit for adults [18].
What the evidence doesn't show
It doesn't show long-term safety, safety in pregnancy or childhood, or whether some devices are safer than others.
- Small, short trials. The trials above enrolled roughly 14 to 60 people for up to 48 weeks. Trials this size can miss side effects that affect only a small share of users.
- Patchy side-effect reporting. One trial asked about specific symptoms [6]. Others reported no adverse events without describing how they checked [2, 3], and some sports trials didn't report collecting them at all [10, 23]. Open-label studies can't separate the effects of hydrogen from the extra water or chance [1, 4].
- Extra water muddies the picture. In one trial, the protocol had people drink about 300 mL more water a day than usual, in both groups [9].
- Industry ties. Some trials were funded or supplied by companies that sell hydrogen products [6]. A 2024 systematic review concluded hydrogen-rich water is "mostly considered safe, with no to minimal side effects," while flagging possible commercial bias and a focus on short-term outcomes [22]. We're funded by a hydrogen-water company too; our methodology explains how we weigh studies, and is hydrogen water real or fake? covers the benefit side.
Frequently asked questions
Does hydrogen water have side effects?
Occasionally, and the ones reported have been mild. Human trials have recorded loose stools, more frequent bowel movements, heartburn, headache, dizziness and more frequent urination. None of the trials we reviewed reported a serious adverse event attributed to hydrogen water, but they were small and short.
Is it safe to drink hydrogen water every day?
Most trials tested daily drinking for periods of 4 to 48 weeks and did not report serious problems. There are no studies of daily use over many years. If you're pregnant, have kidney disease, or take insulin or other glucose-lowering drugs, ask your clinician first.
Can you drink too much hydrogen water?
No upper limit has been set for the hydrogen itself. The practical limits are the amount of water (trials used about 0.5 to 2 liters a day) and, with magnesium tablets, the magnesium they add. For adults, the upper limit for magnesium from supplements is 350 mg a day.
Is hydrogen water safe during pregnancy?
Nobody knows yet. We found no human trials in pregnant or breastfeeding women; trials have excluded them, and the pregnancy research we found is in animals. Talk with your obstetric clinician before using it.
Can hydrogen water cause diarrhea?
It may for some people. In one 8-week study without a control group, 3 of 20 participants had loose stools judged possibly related, and in a placebo-controlled trial one person reported more frequent bowel movements. Magnesium tablets also add magnesium, and the upper limit for supplemental magnesium was set because of diarrhea.
Sources
- Nakao A, et al. Effectiveness of hydrogen rich water on antioxidant status of subjects with potential metabolic syndrome — an open label pilot study. Journal of Clinical Biochemistry and Nutrition. 2010. PubMed · Our summary
- LeBaron TW, et al. The effects of 24-week, high-concentration hydrogen-rich water on body composition, blood lipid profiles and inflammation biomarkers in men and women with metabolic syndrome: a randomized controlled trial. Diabetes, Metabolic Syndrome and Obesity. 2020. PubMed · Our summary
- Kura B, et al. Biological effects of hydrogen water on subjects with NAFLD: a randomized, placebo-controlled trial. Antioxidants. 2022. PubMed
- Ishibashi T, et al. Consumption of water containing a high concentration of molecular hydrogen reduces oxidative stress and disease activity in patients with rheumatoid arthritis: an open-label pilot study. Medical Gas Research. 2012. PubMed · Our summary
- Ito M, et al. Open-label trial and randomized, double-blind, placebo-controlled, crossover trial of hydrogen-enriched water for mitochondrial and inflammatory myopathies. Medical Gas Research. 2011. PubMed · Our summary
- Todorovic N, et al. The effects of 8-week hydrogen-rich water consumption on appetite, body composition, sleep quality, and circulating glucagon-like peptide-1 in obese men and women (HYDRAPPET): a randomized controlled trial. Medicina. 2025. PubMed · Our summary
- Zhang F, et al. Does hydrogen-rich water reduce oxidative stress in patients with chronic mountain sickness? A randomized, blinded, controlled trial. High Altitude Medicine & Biology. 2025. PubMed · Our summary
- Yoritaka A, et al. Pilot study of H₂ therapy in Parkinson's disease: a randomized double-blind placebo-controlled trial. Movement Disorders. 2013. PubMed · Our summary
- Sim M, et al. Hydrogen-rich water reduces inflammatory responses and prevents apoptosis of peripheral blood cells in healthy adults: a randomized, double-blind, controlled trial. Scientific Reports. 2020. PubMed · Our summary
- Valenta M, et al. Acute pre-exercise hydrogen rich water intake does not improve running performance at maximal aerobic speed in trained track and field runners: a randomized, double-blind, placebo-controlled crossover study. PLoS One. 2022. PubMed · Our summary
- Ohno K, et al. Molecular hydrogen as an emerging therapeutic medical gas for neurodegenerative and other diseases. Oxidative Medicine and Cellular Longevity. 2012. PubMed · Our summary
- Gardette B, Delauze HG. Techniques of underwater intervention: means, methods, research and outlook [article in French]. Bulletin de l'Académie Nationale de Médecine. 1996. PubMed
- Abraini JH, et al. Psychophysiological reactions in humans during an open sea dive to 500 m with a hydrogen-helium-oxygen mixture. Journal of Applied Physiology. 1994. PubMed · Our summary
- Saitoh Y, et al. Biological safety of neutral-pH hydrogen-enriched electrolyzed water upon mutagenicity, genotoxicity and subchronic oral toxicity. Toxicology and Industrial Health. 2010. PubMed · Our summary
- US Food and Drug Administration. GRAS Notice No. 520: Hydrogen gas (FDA response: "no questions," November 28, 2014). GRAS Notices inventory. hfpappexternal.fda.gov
- NOAA Office of Response and Restoration. Hydrogen — chemical datasheet. CAMEO Chemicals. cameochemicals.noaa.gov
- Cole AR, et al. Safety of prolonged inhalation of hydrogen gas in air in healthy adults. Critical Care Explorations. 2021. PubMed · Our summary
- NIH Office of Dietary Supplements. Magnesium — fact sheet for health professionals. ods.od.nih.gov
- Costello R, et al. Perspective: call for re-evaluation of the tolerable upper intake level for magnesium supplementation in adults. Advances in Nutrition. 2023. PubMed
- Imai K, et al. Neuroprotective potential of molecular hydrogen against perinatal brain injury via suppression of activated microglia. Free Radical Biology and Medicine. 2016. PubMed · Our summary
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Eating & nutrition for hemodialysis. www.niddk.nih.gov
- Dhillon G, et al. Hydrogen water: extra healthy or a hoax? — A systematic review. International Journal of Molecular Sciences. 2024. PubMed · Our summary
- Sládečková B, et al. Hydrogen-rich water supplementation promotes muscle recovery after two strenuous training sessions performed on the same day in elite fin swimmers: randomized, double-blind, placebo-controlled, crossover trial. Frontiers in Physiology. 2024. PubMed · Our summary
How we wrote this: drafted with AI assistance from the sources above and checked against each source by the Hydrogen Studies editorial team. This is general information, not medical advice — talk to your clinician before changing any treatment.