In short: Epsom salt is magnesium sulphate, a salt made of magnesium and sulfuric acid. From the pharmacy, it's a tried-and-tested laxative: 10 to 15 g, dissolved in a glass of water, usually clears the bowel within about six hours. It's meant for short-term use. In small amounts, magnesium sulphate also serves as a magnesium source in food supplements.
Key points
- Two names, one substance: Epsom salt, magnesium sulphate and the US spelling "magnesium sulfate" mean the same salt. It has a distinctly bitter taste.
- Tried and tested for constipation: as a medicine, it helps short-term with constipation and with clearing the bowel, for example before an examination.
- High in magnesium: Epsom salt is 9.9% magnesium. A laxative dose of 10 g provides around 990 mg (our calculation).
- Also permitted as a food supplement: magnesium sulphate is on the EU's list of magnesium compounds. In a 24-week study, it raised blood magnesium just as well as magnesium citrate.
- Not for long-term use, and not with weak kidneys: Epsom salt is meant for a few days at most. With reduced kidney function, it's off limits.
Contents
- What is Epsom salt?
- How much magnesium is in Epsom salt?
- How does Epsom salt work in the gut?
- Taking Epsom salt: dosage and use
- When should you not take Epsom salt?
- Epsom salt for fasting and "liver cleanses"
- Magnesium sulphate as a food supplement
- Epsom salt baths and magnesium sulphate in hospital
- Frequently asked questions
- Limits of the research and all sources
What is Epsom salt?
Epsom salt is the common name for magnesium sulphate. Usually what's meant is the form with water of crystallisation, chemically magnesium sulphate heptahydrate. It looks like coarse, clear salt and dissolves easily in water.
The same substance is sold for very different purposes. What matters for you is what the particular product is meant for:
| Where you'll find it | What it's for | Safe to take internally? |
|---|---|---|
| Pharmacy, as a medicine | short-term for constipation, bowel clearance before examinations1 | yes, following the patient information leaflet |
| Ingredient in food supplements | magnesium source, approved by the EU2 | yes, in the stated daily amount |
| Drugstore, as bath salt | additive for a full bath or foot bath | no |
| Garden centre, as fertiliser | magnesium for plants | no |
What the studies show: In Germany, Epsom salt from the pharmacy is a licensed medicine with a patient information leaflet. It lists the short-term treatment of constipation and bowel clearance before examinations as its approved uses.1 As an ingredient in food supplements, magnesium sulphate is on the EU's list of permitted magnesium compounds.2
How much magnesium is in Epsom salt?
Epsom salt is 9.9% magnesium. So 10 g of Epsom salt contains around 990 mg of magnesium. Anhydrous magnesium sulphate contains twice as much, 20.2%, because it has no water of crystallisation.
| Form | Magnesium content | Magnesium in 10 g |
|---|---|---|
| Epsom salt (magnesium sulphate heptahydrate) | 9.9% | around 990 mg |
| Magnesium sulphate, anhydrous | 20.2% | around 2,020 mg |
Our calculation from the molar masses.3
How we calculated this: Magnesium weighs 24.31 units, Epsom salt 246.47, and anhydrous magnesium sulphate 120.37.3 24.31 divided by 246.47 gives 9.9%, divided by 120.37 gives 20.2%.
For comparison: for food supplements, the EU food safety authority recommends no more than 250 mg of magnesium a day.10 A laxative dose of Epsom salt contains four to six times that. That's exactly why it acts so strongly on the gut, and why Epsom salt is a short-term medicine rather than something for your daily magnesium supply. How much magnesium you need per day is explained in How much magnesium per day?
How does Epsom salt work in the gut?
Epsom salt draws water into the gut. The body absorbs both magnesium and sulphate poorly, so both stay in the gut and hold water there. The stool becomes looser and bulkier, and the bowel empties.
What the studies show: A review describes exactly this mechanism. Messenger substances that the gut releases in response to Epsom salt probably add to this, stimulating it further.4 Sulphate itself is also only absorbed to a limited extent, because the body can only take up a certain amount.5 In one study, six healthy men swallowed almost 14 g of Epsom salt spread over three hours. Over the following 24 hours, only around 4% of the magnesium showed up again in their urine, and all of them had mild to moderate diarrhoea.6
How well does this work against constipation? A systematic review of over-the-counter laxatives rates the evidence for magnesium salts as moderate, at the same level as psyllium husk or prunes.7 Mineral water rich in magnesium sulphate has also been studied: 244 women with constipation drank 1 litre of it daily for four weeks. After two weeks, constipation had improved in 37.5%, compared with 21.1% for plain water.8
Tip: If you have occasional, mild constipation, a mineral water rich in magnesium and sulphate may already be enough. One litre of it contains far less magnesium sulphate than a dose of Epsom salt. The figures are on the label under "magnesium" and "sulphate".
Taking Epsom salt: dosage and use
Adults dissolve 2 to 3 teaspoons of Epsom salt (10 to 15 g) in about 250 ml of water and drink it. That's what the patient information leaflet says. For a quick, thorough bowel clearance, it's 1 to 2 tablespoons (20 to 30 g) in about 500 ml of water.1 The effect usually sets in within about six hours.9
How to do it:
- Use Epsom salt from the pharmacy, not bath salt or fertiliser.
- Dissolve the dose completely in a large glass of water. If you don't like the bitter taste, add a little lemon juice or rinse with juice afterwards.
- It's best taken in the morning; stay near a toilet for the next few hours.
- Drink plenty afterwards, because water and salts are lost along with the stool.
- For children, only after talking to a GP or paediatrician.
How long can you take Epsom salt for? Only briefly: once, or on a few days at most. The patient information leaflet warns that longer use can make the bowel more sluggish. With prolonged use, water, potassium and other salts are also lost.1 If constipation lasts longer, have the cause checked by a doctor.
When should you not take Epsom salt?
Healthy adults generally tolerate a single dose as directed well, apart from the intended diarrhoea and sometimes stomach cramps or nausea.9 For some people, though, Epsom salt isn't suitable.
Important: Don't take Epsom salt if you have
- reduced kidney function,
- bowel obstruction, unexplained abdominal pain or inflammatory bowel disease,
- disturbances in your water or salt balance.1, 9
There are interactions with medicines. Certain antibiotics (tetracyclines, quinolones) are absorbed less well by the body; leave a gap of 2 to 4 hours.9 If you take diuretics ("water tablets"), corticosteroids or cardiac glycosides, you lose more potassium under Epsom salt. Discuss this with your GP first.1
Why the kidneys specifically? A small part of the magnesium always ends up in the blood, and the kidneys excrete it again. If they're not working well, magnesium can build up.11 We explain more in Too much magnesium.
What the studies show: Too much magnesium in the blood from Epsom salt is rare but well documented. Typical signs are muscle weakness, drowsiness, low blood pressure and heart rhythm disturbances.1, 12 In a hospital review spanning five years, 7 of 8 affected patients had a condition of the stomach or gut, such as an ulcer or inflammation, that let more magnesium through. Only one had pre-existing severe kidney weakness.12 Case reports also describe severe courses with normal kidneys, for instance after an accidental overdose or in a patient with stomach surgery and stomach bleeding.13, 14 You'll find how to spot too much magnesium in Symptoms of too much magnesium.
Epsom salt for fasting and "liver cleanses"
Many people know Epsom salt from fasting. At the start of Buchinger-style therapeutic fasting, the bowel is cleared with a laxative salt, usually Glauber's salt (sodium sulphate) or Epsom salt. For healthy adults, a single bowel clearance like this, following the product directions, is easily manageable.
What the studies show: In the largest observational study of Buchinger fasting, with 1,422 participants at a fasting clinic, the fast began with 20 to 40 g of Glauber's salt in 500 ml of water. An enema or a mild laxative then followed every other day. Fewer than 1% of participants reported side effects.15 Whether the bowel clearance itself contributes to the benefit of fasting wasn't specifically studied.
It's a different picture with the so-called liver or gallbladder cleanse. Here, people drink Epsom salt and, in the evening, a large amount of olive oil with lemon juice. The next day, greenish pellets appear in the stool, said to be flushed-out gallstones.
What the studies show: A laboratory team examined pellets like these from one patient. They consisted mainly of fatty acids and contained no cholesterol, bile pigment or calcium — none of what gallstones are made of. Very similar pellets could be produced in the lab from oleic acid and lemon juice.16 A case report describes a man with gallstones who, on a naturopath's advice, drank three tablespoons of Epsom salt daily for 15 days. He developed severe liver damage, which recovered within 38 days after stopping.17
Our take: To start a fast, a single dose of Epsom salt is a well-established way for healthy adults to clear the bowel. Stick to the directions and drink plenty afterwards. For a liver cleanse, you don't need Epsom salt at all: the liver does its job without any cleanse, and the pellets after an oil cleanse aren't gallstones. If you have gallstones, the best approach is to discuss treatment with a doctor. And the weight that's missing from the scale after Epsom salt is mostly water and bowel contents.
Always buy Epsom salt for internal use from a pharmacy. In one documented case, several people became severely ill and one died because a supplier had mistakenly filled a liver-cleanse product with manganese sulphate instead of magnesium sulphate.18
Magnesium sulphate as a food supplement: how well is it absorbed?
Well, in small amounts spread across the day. Magnesium sulphate is approved in the EU as a magnesium source for food supplements.2 In one study, it raised blood magnesium over half a year just as well as magnesium citrate.19 So it delivers magnesium in the normal way, and for this, these are among the officially confirmed claims in the EU:
- Magnesium contributes to electrolyte balance.
- Magnesium contributes to normal muscle function.
- Magnesium contributes to normal functioning of the nervous system.
What magnesium does in the body is explained in Magnesium benefits.
What the studies show: In a controlled study, 164 adults took 450 mg of magnesium a day for 24 weeks, as citrate, oxide or sulphate, or a placebo. Blood magnesium rose similarly in all three groups. More magnesium showed up in urine after citrate than after oxide or sulphate.19 A systematic review of 14 comparative studies found overall that organic forms like citrate are absorbed somewhat better than inorganic ones.20
Our take: Magnesium sulphate in a food supplement is an approved, effective magnesium source, as long as the daily amount stays small. Split larger amounts across two meals. If you have a sensitive stomach, you may find an organic form more comfortable, because sulphate itself holds water in the gut. For an overview of all forms, see Which magnesium is best?
Epsom salt baths and magnesium sulphate in hospital
Epsom salt bath: Many people use Epsom salt externally as a bath additive, for a full bath or a foot bath in the evening. The warm water feels good and can make it easier to fall asleep. How much magnesium actually gets through the skin this way, and how to use the bath properly, is covered in our article Magnesium oil, spray and baths. Bath salt isn't meant for drinking.
Infusion in hospital: In hospital, magnesium sulphate is also given as an infusion into a vein, for instance for certain complications in pregnancy. There, it's considered the treatment of choice.21 This is a medical therapy with close monitoring of blood levels, and has nothing to do with Epsom salt for drinking or with food supplements.
Frequently asked questions
Is Epsom salt the same as magnesium? Epsom salt is a magnesium compound: magnesium, bound to sulphate, with water of crystallisation. Just under a tenth of it is magnesium.3 As a laxative, it works because of the large single dose that the gut can't absorb.
How fast does Epsom salt work? Usually within about six hours.9 Taken in the morning, the effect comes on over the course of the morning or early afternoon.
Can I take Epsom salt every day? Not as a laxative. It's meant for short-term use.1 For your everyday magnesium supply, food and food supplements with no more than 250 mg of magnesium a day are the better fit.10
Can I drink Epsom salt meant for plants, or bath salt? No. This kind of product hasn't been tested as a medicine. Only Epsom salt from the pharmacy is meant for drinking.1, 18
What is magnesium sulphate paste, and can you leave it on overnight? A paste of magnesium sulphate and glycerin, known especially in the UK as a home remedy for splinters and minor skin inflammations such as boils. There are no studies on this, so there's also no evidence on how long it can safely be left on; follow the directions on the pack. The mixture itself has been studied in hospitals for vein irritation caused by cannulas, with early positive results, mostly from small studies.22 If the skin is red, hot or painful, have the area looked at by a doctor.
Can I take Epsom salt during pregnancy? According to the patient information leaflet, no harmful effects are known during pregnancy and breastfeeding.1 Even so, discuss constipation in pregnancy with your midwife or GP first. More on magnesium during this time is in Magnesium during pregnancy.
Limits of the research and all sources
The laxative effect of Epsom salt has been known and well documented for a long time, but studies specifically on Epsom salt as a powder are rare. The review of over-the-counter laxatives rates magnesium salts as a group.7 The mineral water study tested one manufacturer's water, and the review built on it involved a consultancy.8 On absorption from Epsom salt, there's only one small, older study with six men that measured urine alone.6 The study on magnesium sulphate as a food supplement was co-funded by a magnesium raw-material manufacturer. There, the sulphate group reported more gastrointestinal complaints after 24 weeks than the placebo group, but not more than the citrate group.19 The severe cases come from case reports. They show what can happen, but not how often it does.12, 13, 14, 17 The fasting study was an observational study with no comparison group, funded by the fasting clinic where it took place.15 That longer use makes the bowel more sluggish is a note from the patient information leaflet; there are no dedicated long-term studies on Epsom salt for this.1
Show all 22 sources
| # | Study | Who was studied | Result | Strength of evidence |
|---|---|---|---|---|
| 1 | Patient information leaflet, Epsom salt (magnesium sulfate 7H2O), Caesar & Loretz, as of 08/2014 | medicine information | short-term for constipation; 10–15 g in 250 ml, rapid 20–30 g in 500 ml; not for kidney weakness, bowel obstruction; interactions, overdose | officially reviewed product information for a German pharmacy Epsom salt |
| 2 | Directive 2002/46/EC, Annex II | EU law | magnesium sulphate listed as a permitted magnesium compound | legal basis |
| 3 | PubChem: Magnesium sulfate CID 24083, Magnesium sulfate heptahydrate CID 24843 | substance database | 120.37 and 246.47 g/mol → 20.2% and 9.9% magnesium | reference data, our calculation |
| 4 | Izzo et al. 1996, Magnes Res | review | magnesium and sulphate osmotically hold water in the gut; messenger substances and nitric oxide also play a part | review, mechanism partly from animal studies |
| 5 | Moulin et al. 2018, Endosc Int Open | review of sulphate laxative solutions | sulphate absorption is limited (saturable) | review |
| 6 | Morris et al. 1987, J Toxicol Clin Toxicol | 6 healthy men, 13.9 g Epsom salt | 4.0% of the magnesium in 24 h, 6.9% in 72 h, in urine; all with diarrhoea | very small study, urine measurement only |
| 7 | Rao & Brenner 2021, Am J Gastroenterol | 41 studies on over-the-counter laxatives | magnesium salts: moderate evidence (grade B) | systematic review, magnesium salts as a group |
| 8 | Dupont et al. 2014, Clin Gastroenterol Hepatol | 244 women with constipation, 4 weeks | 1 l magnesium-sulphate-rich mineral water: after 2 weeks 37.5% vs 21.1% improved; review: Dupont & Hébert 2020 | controlled study; one manufacturer's water was tested, funding not stated in the abstract; review co-authored with a consultancy |
| 9 | PharmaWiki: Epsom salt, as of 05/2026 | medicine reference work | effect within about 6 h; contraindications; 2–4 h gap from antibiotics | reference information |
| 10 | EFSA / SCF 2001 | agency assessment | upper limit 250 mg/day from food supplements | official assessment |
| 11 | Rosner et al. 2023, Mayo Clin Proc | review | magnesium build-up with reduced kidney function and magnesium medicines | review |
| 12 | Clark et al. 1992, Am J Nephrol | 8 hospital cases over 5 years | severe hypermagnesaemia from laxatives or stomach remedies; 7 of 8 with a stomach or gut condition, only 1 with pre-existing kidney weakness | case series |
| 13 | Si et al. 2024, Front Med | 1 patient, Epsom salt before colonoscopy | life-threateningly high magnesium level despite normal kidneys; stomach surgery and stomach bleeding | case report |
| 14 | Khan et al. 2018, Caspian J Intern Med | 1 patient, accidental overdose | fatal outcome despite normal kidney function | case report |
| 15 | Wilhelmi de Toledo et al. 2019, PLoS ONE | 1,422 people fasting, 4–21 days Buchinger fasting | started with 20–40 g Glauber's salt; side effects < 1% | observational study with no comparison group; funded by the fasting clinic, authors employed there |
| 16 | Sies & Brooker 2005, Lancet | "stones" from one patient after a liver cleanse | fatty acids, no cholesterol, bilirubin or calcium; reproduced from oleic acid + lemon juice | case description with lab analysis |
| 17 | Philips et al. 2017, BMJ Case Rep | 1 patient, 15 days, 3 tbsp Epsom salt daily | severe liver damage, recovery after 38 days; link assessed as probable | case report, pre-existing liver condition |
| 18 | Sánchez et al. 2012, Forensic Sci Int | several patients, liver-cleanse regimen | supplier filled manganese sulphate instead of Epsom salt; one death | case report |
| 19 | Schutten et al. 2022, J Am Heart Assoc | 164 adults, 450 mg/day, 24 weeks | blood level similar for citrate, oxide, sulphate; urine: citrate ahead; sulphate more gastrointestinal complaints than placebo (24 wks) | large controlled study, co-funded by a magnesium raw-material manufacturer |
| 20 | Pardo et al. 2021, Nutrition | 14 comparative studies | organic forms absorbed better than inorganic | systematic review |
| 21 | Duley et al. 2010, Cochrane | 15 studies, 6 with 11,444 women with pre-eclampsia | magnesium sulphate infusion more than halves the risk of seizures | Cochrane review, medical therapy |
| 22 | García-Expósito et al. 2022, J Clin Nurs | 22 studies, 2,042 hospital patients | for vein irritation from cannulas, magnesium sulphate with glycerin performed best | scoping review, study quality often limited |
Author: PRONESIUM Editorial Team · Last reviewed: 01/10/2026 · This article does not replace medical advice.
