The short answer
In trials that reported adverse events, hydrogen water itself has not shown severe side effects: an 8 week randomized controlled trial in people with obesity reported no severe adverse effects1 and a hyperuricemia trial in 100 participants called it a rather safe agent2. But most trials never reported adverse events, so this is not proof of safety. The documented side effect risk sits with the magnesium in hydrogen tablets, which can loosen stools3, and with groups nobody has studied.
What did human trials of hydrogen water record as side effects?
Only two of the trials we reviewed state adverse events in their abstracts. An 8 week randomized controlled trial in people with obesity reported no severe adverse effects throughout1. A trial in 100 participants with hyperuricemia who drank three bottles of hydrogen rich water daily for 8 weeks called it a rather safe agent2.
Most other studies simply say nothing. A meta-analysis of 27 publications with 597 participants did not record adverse events as an outcome4. A trial with 99 non-trained and 60 trained participants drinking hydrogen water before exercise did not report adverse events in the abstract5. A 3 month trial in 181 subjects with metabolic or pre-metabolic syndrome also did not6.
Missing reporting is not evidence of absence. Absence of reported side effects in small, short trials tells you only that nothing dramatic happened to a few hundred people over a few weeks.
Is the hydrogen gas itself a risk at drinking-water levels?
No trial in this review linked dissolved hydrogen at drinking water levels to a side effect, but only two trials reported adverse events at all. Hydrogen is a dissolved gas that leaves the water quickly, much like the bubbles in sparkling water, and it is not a medicine.
None of the reviewed trials were designed to detect rare or long term harms. The largest single trial here enrolled 181 subjects for 3 months6. That is a small evidence base for statements about safety in everyone, forever.
What does a hydrogen tablet add to your glass besides hydrogen?
A hydrogen tablet is not just hydrogen. Our Molecular Hydrogen Tablets also add minerals and excipients per tablet, and the label maximum is 2 tablets per day, which doubles every figure below.
| Ingredient | Per tablet | At label maximum of 2 tablets | What it is there for |
|---|---|---|---|
| Magnesium | 80 mg (21% NRV) | 160 mg | Reacts with water to release hydrogen gas; authorised claims: magnesium contributes to electrolyte balance and to a reduction of tiredness and fatigue |
| Potassium | 16 mg (0.8% NRV) | 32 mg | Mineral component of the reaction mixture |
| Sodium | 41 mg | 82 mg | Mineral component of the reaction mixture |
| Glucose, mannitol | Not stated per tablet | Not stated | Bulk and binding agents |
| Citric acid, malic acid | Not stated per tablet | Not stated | Acids that start and speed up the effervescent reaction |
So the side effect question splits in two: hydrogen gas per the trials above, and these added substances. Of these, magnesium is the one with documented gastrointestinal side effects in trials. For context on minerals that are already in your water versus minerals from a tablet, see our article on minerals, hardness and taste in water.
Can the magnesium in hydrogen tablets upset your stomach?
Yes, magnesium salts can loosen stools, and that is the most documented side effect of oral magnesium. In Cochrane cramp trials, oral magnesium was associated with mostly gastrointestinal adverse events such as diarrhoea, experienced by 11% to 37% of participants versus 10% to 14% in control groups3.
More minor adverse events occurred with magnesium than placebo overall (RR 1.51, 95% CI 0.98 to 2.33 across 4 studies with 254 participants), while major adverse events were not significantly different from placebo (2 out of 72 magnesium recipients versus 3 out of 68 placebo recipients)3.
Two tablets at the label maximum give 160 mg magnesium per day. For comparison, the US Institute of Medicine set a tolerable upper intake level of 350 mg per day from supplemental magnesium alone in 1997, with diarrhoea as the limiting factor; this is the US figure, not an EU limit7. That said, a 2023 re-evaluation found that seven intervention studies with magnesium intakes of 128 to 1200 mg per day showed no significant differences in diarrhoea occurrence between intervention and control groups, so the US figure is debated7.
Should people with kidney problems be careful with magnesium tablets?
Yes. Impaired kidneys clear magnesium less well, and reviews describe overt renal failure as practically the only contraindication for oral magnesium supplementation at physiological doses of 5 mg per kg per day8. One case report describes fatal hypermagnesemia in an elderly woman who was given a magnesium-containing laxative while she had unrecognised acute renal failure9. A review of magnesium oxide likewise asks for attention to dose, blood levels, drug interactions and side effects, especially in older people and in people with renal impairment10.
Our label reflects this in general wording: consult a doctor with a medical condition, in pregnancy or while breastfeeding. If your kidney function is reduced, that conversation is not optional. Note the reviews also warn that high oral doses above 10 mg per kg per day have a laxative action and may bring latent complications8.
Does a hydrogen water bottle add anything you should know about?
Potentially, yes: electrolysis bottles can produce chlorine as a by-product. In one published test, bottles from three companies reached 0.2 to 1.0 mg/L chlorine after 10 to 30 minutes, depending on the model, all under the 5 mg/L WHO guideline used by the authors. We covered this fully in hydrogen water tablets or a hydrogen water bottle, so we will not repeat it here.
What has nobody studied yet?
Plainly: children, pregnancy, breastfeeding, years of daily use and interactions with medication. The reviewed trials were short, from 8 weeks to 3 months, and enrolled adults. No EU authorised health claim exists for molecular hydrogen.
Why do critics call hydrogen water overhyped?
Because the evidence base is small, short and mixed. Meta-analyses of hydrogen and performance found trivial, non-significant effects on VO2max and endurance4, and trials cover a few hundred people at a time. Few side effects reported is not the same as proven benefit: those are two separate questions and critics are right to keep them apart.
Frequently Asked Questions
Does hydrogen water have documented side effects?
Not in the trials that reported them: one trial reported no severe adverse effects1 and another called it a rather safe agent2. Most trials did not report adverse events at all, which is not proof of safety.
Can hydrogen water cause diarrhoea?
The hydrogen gas itself has not been linked to it in trials. Magnesium from tablets can: gastrointestinal events affected 11% to 37% of oral magnesium users in cramp trials3.
Is 160 mg of magnesium per day a lot?
It is below the US Institute of Medicine upper level of 350 mg per day for supplemental magnesium, which is the US figure, not an EU limit7. Individual tolerance varies.
Is hydrogen water safe in pregnancy or for children?
Nobody knows; no trials cover these groups. Our label says to consult a doctor in pregnancy, breastfeeding or with a medical condition.
Are there long term studies of daily hydrogen water?
Not really: the trials we reviewed ran from 8 weeks to 3 months6. Years of daily use have not been studied.
The Bottom Line
Hydrogen water itself looks unremarkable for side effects in the small, short trials that exist, and two trials explicitly reported it as safe12. The honest answer is that the real documented side effect risk is the magnesium in tablets, mainly loose stools, and that nobody has studied children, pregnancy or long term daily use. Read more in our article on molecular hydrogen, and follow the label: maximum 2 tablets per day.
Sources
- 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 (Kaunas), 2025
- Effects of hydrogen-rich water on blood uric acid in patients with hyperuricemia: A randomized placebo-controlled trial, Heliyon, 2024
- Magnesium for skeletal muscle cramps, The Cochrane database of systematic reviews, 2020
- Can molecular hydrogen supplementation enhance physical performance in healthy adults? A systematic review and meta-analysis, Frontiers in nutrition, 2024
- Drinking hydrogen water enhances endurance and relieves psychometric fatigue: a randomized, double-blind, placebo-controlled study, Canadian journal of physiology and pharmacology, 2019
- Health Effects of Electrolyzed Hydrogen Water for the Metabolic Syndrome and Pre-Metabolic Syndrome: A 3-Month Randomized Controlled Trial, Antioxidants (Basel), 2024
- Perspective: Call for Re-evaluation of the Tolerable Upper Intake Level for Magnesium Supplementation in Adults, Advances in nutrition, 2023
- Magnesium and therapeutics, Magnesium research, 1994
- Fatal hypermagnesemia, Clinical nephrology, 2000
- Magnesium Oxide in Constipation, Nutrients, 2021
