Best Magnesium for Constipation UK

Constipation is one of the most common reasons people walk into a UK pharmacy. NHS estimates suggest around 1 in 7 adults and up to 1 in 3 children in the UK are constipated at any one time, and the condition affects roughly twice as many women as men. The Bowel Interest Group's Cost of Constipation analysis put NHS England's annual spend at approximately £168 million, including around £87 million on laxative prescriptions and nearly 77,000 hospital admissions where constipation was the final diagnosis.

Magnesium sits in an unusual space in that landscape. It is simultaneously an essential mineral, a licensed medicine and a supplement sold on every high street — and the form you buy determines almost entirely whether it will move your bowels, correct a dietary shortfall, or do neither. This guide separates those outcomes, ranks the magnesium products actually available in the UK, and sets out the doses, timings and safety limits that matter.

If constipation is a long-standing pattern rather than an occasional inconvenience, read this alongside our wider guide to constipation relief in the UK, our overview of gut health in the UK and our dedicated article on magnesium for constipation. Our gut health range and probiotics collection cover the other components of a fix.

Quick answer: which magnesium is best for constipation?

Start by deciding whether you want a bowel movement tonight or better regularity over the coming weeks — the two goals need different magnesium forms.

Best magnesium supplement overall: the Welzo Ultra Purity Magnesium L-Threonate 2000mg. On the criteria that determine whether a magnesium supplement is worth taking at all — verified potency, third-party testing, GMP manufacture, published Certificate of Analysis, full dose disclosure and no fillers — nothing else in the UK range matches it. Because L-threonate is highly absorbed, it raises magnesium status rather than acting as a laxative.

Fastest relief: magnesium hydroxide, sold in the UK as Milk of Magnesia. It is the only magnesium preparation licensed here specifically for constipation, it is poorly absorbed (exactly what you want in a laxative), and it typically works within 30 minutes to 6 hours.

Best evidence for chronic constipation: magnesium oxide, the form used in the two Japanese placebo-controlled trials and named in the 2023 joint AGA/ACG guideline.

Best dual-purpose: magnesium citrate — absorbed well enough to correct low intake, unabsorbed enough at higher doses to soften stool.

Best magnesium for constipation UK: top picks at a glance

Prices correct at time of writing and subject to change — check the product page. Browse the full Welzo magnesium collection.

How we chose (our ranking methodology)

Because constipation is a health decision rather than a lifestyle purchase, we ranked against clinical and manufacturing criteria rather than marketing claims.

1. Manufacturing standard, testing and transparency

This is the criterion that decides whether a magnesium supplement is worth taking at all, and it is why the Welzo Ultra Purity magnesium ranks first. It is third-party tested for identity, potency and contaminants, manufactured in a GMP-certified facility, published with a downloadable Certificate of Analysis, disclosed in full with no proprietary blends, and formulated without unnecessary fillers, binders, artificial colours or sweeteners. Very few supplement brands meet all five of those conditions at once.

2. Salt form and its match to the goal

The anion attached to magnesium determines almost everything. We separated products into laxative-first (hydroxide, oxide, sulphate), dual-purpose (citrate) and absorption-first (threonate, glycinate, malate, taurate), and only claim a bowel effect where the chemistry supports one.

3. Elemental magnesium per serving

A "500 mg magnesium oxide" capsule does not deliver 500 mg of magnesium. Oxide is roughly 60% elemental magnesium by weight; citrate is closer to 11–16%. We compared on elemental content, not compound weight.

4. Strength of the human evidence

Forms supported by randomised controlled trials in constipation were weighted above forms supported only by mechanism or anecdote.

5. Titratability

The correct dose is the smallest one that produces a soft, formed stool. Powders and liquids that allow small increments scored above fixed-dose tablets.

6. UK availability and cost per effective dose

We prioritised reliable UK dispatch and calculated price per gram of elemental magnesium rather than price per bottle.

The 11 best magnesium supplements for constipation in the UK

1. Welzo Ultra Purity Magnesium L-Threonate 2000mg — Best Magnesium Supplement Overall

Welzo Ultra Purity Magnesium L-Threonate 2000mg delivers 2000 mg of magnesium L-threonate per three-capsule serving in vegetarian capsules, with a downloadable Certificate of Analysis and a clean excipient profile. Rated 5 stars across 94 reviews, currently £19.99 (reduced from £39.99).

Why it ranks first

Supplement quality is decided long before the label copy. On verified potency, third-party testing, GMP manufacture, full dose disclosure, absence of fillers and a published COA, this is the strongest magnesium product available in the UK range — and the one we would put anyone on as a daily magnesium. L-threonate is a highly bioavailable form, well studied for raising magnesium levels centrally, making it the right choice where the priority is correcting magnesium status, supporting sleep, and supporting normal nervous system and muscle function.

How it fits into a constipation plan

Be clear about the role it plays. Magnesium L-threonate is designed to be absorbed in the small intestine, which means comparatively little magnesium remains in the colon to draw in water — so it should not be relied on as a laxative. Where it earns its place is as the daily foundation: correcting a low magnesium intake supports normal smooth-muscle function in the gut wall, and around 19% of UK adults in their twenties have magnesium intakes below the Lower Reference Nutrient Intake. Most people running it for constipation take it nightly alongside an osmotic form for the bowel effect itself.

Pros

• Third-party tested with a published Certificate of Analysis
• GMP-manufactured; clean label, no proprietary blends
• Highly bioavailable form — genuinely raises magnesium status
• Supports sleep and relaxation, which helps gut–brain-driven constipation
• Strong value at the current sale price; UK dispatch; 5 stars across 94 reviews

Cons

• Not a laxative — will not reliably produce a bowel movement
• Lower elemental magnesium per gram than oxide
• Three capsules per serving; 20 servings per container
• Not suitable in pregnancy, breastfeeding or for children

2. Milk of Magnesia 200ml — Fastest Relief and the Only UK-Licensed Magnesium Laxative

Milk of Magnesia contains magnesium hydroxide 415 mg alongside sodium hydrogen carbonate and peppermint oil. It is a liquid oral medicine indicated for indigestion, heartburn, excess wind and constipation — and the only magnesium format in the UK with a licence covering constipation specifically. £8.78 for 200ml, 32 reviews.

Why it is the fastest option

Magnesium hydroxide is poorly absorbed, which maximises the osmotic effect in the bowel while minimising the amount reaching the bloodstream. Onset is typically 30 minutes to 6 hours, so an evening dose commonly produces a morning result. The manufacturer's laxative dose for adults is 30–45 ml at bedtime, reducing nightly until constipation resolves. The liquid format also allows precise dose adjustment, which capsules do not.

Pros

• The only UK-licensed magnesium preparation for constipation
• Fast, predictable onset (30 minutes to 6 hours)
• Easy to titrate; doubles as an antacid
• No stimulant action, so less cramping than senna
• Inexpensive

Cons

• Should not be used for more than one week without medical advice
• Chalky taste; requires shaking before use
• Not for children under 3 without a doctor's advice
• Contains glycerol, which at adult doses can itself cause headache or stomach upset
• No meaningful contribution to nutritional magnesium status

3. Douglas Labs Magnesium Oxide 500 mg — Best Evidence-Based Option for Chronic Constipation

Douglas Labs Magnesium Oxide 500 mg (£21.01, 100 capsules) is the closest UK-available equivalent to the preparation used in the randomised trials.

The evidence behind magnesium oxide

Mori and colleagues published a randomised, double-blind, placebo-controlled trial in the Journal of Neurogastroenterology and Motility in 2019. Thirty-four patients with chronic constipation received either magnesium oxide 0.5 g three times daily (1.5 g/day) or placebo for 28 days. The response rate for overall symptom improvement was 70.6% on magnesium oxide versus 25.0% on placebo. Colonic transit time fell from a mean of roughly 75.5 hours at baseline to 41.6 hours at week four in the treatment arm.

A second trial by Morishita and colleagues in the American Journal of Gastroenterology randomised 90 patients to senna 1.0 g/day, magnesium oxide 1.5 g/day or placebo for 28 days; both active treatments improved symptoms.

On this basis the 2023 joint AGA/ACG guideline issued a conditional recommendation for magnesium oxide in adults with chronic idiopathic constipation, rating the certainty of evidence as very low. Guideline co-chair Dr Lin Chang of UCLA noted the recommendations are intended to provide a framework for approaching the management of CIC, with shared decision-making around patient preference, cost and availability.

Pros

• Highest elemental magnesium per capsule (~60%)
• The form used in the published randomised trials
• Named in an international gastroenterology guideline
• Inexpensive per dose

Cons

• Poorly absorbed — unsuitable for correcting magnesium deficiency
• Effect depends partly on gastric acid, so may be blunted on PPIs
• Bloating and cramping common at higher doses
• Trial doses exceed typical supplement labelling — discuss with a clinician first

4. Viridian Magnesium Citrate Powder 150g — Best Dual-Purpose Citrate

Viridian Magnesium Citrate Powder (£22.29, 80 reviews) is 100% active magnesium citrate with no fillers, produced to vegan, non-GMO and palm-oil-free standards. Half a level teaspoon provides 375 mg of elemental magnesium.

Why citrate sits in the sweet spot

Citrate is absorbed well enough to raise magnesium status but remains unabsorbed enough at higher doses to hold water in the colon. As a powder it is the easiest form to titrate — you can start at a quarter teaspoon and work up in small steps until stools soften.

Pros

• Single-ingredient formula, no bulking agents or flow aids
• Strong UK brand ethics and transparency record
• Highly titratable; genuinely dual-purpose
• Better absorbed than oxide, so corrects intake shortfalls too

Cons

• Tart taste, unmasked by flavouring
• Easy to overshoot into loose stools without careful measuring
• Frequently out of stock — check availability

5. Swanson Triple Magnesium Complex 400 mg — Best Value

Swanson Triple Magnesium Complex blends oxide, citrate and aspartate, giving a poorly absorbed fraction for bowel effect alongside better-absorbed fractions for magnesium status. At 300 capsules for £13.27 it is one of the cheapest routes to a therapeutic dose in the UK.

Pros

• Three complementary salt forms in one capsule
• Outstanding cost per gram of elemental magnesium
• 300-capsule bottle lasts months

Cons

• Blend ratios not broken down per form on the label
• Fixed capsule dose limits fine titration
• Laxative effect less predictable than a single-salt product

6. Solaray Magnesium Glycinate 350 mg — Best Gentle Daily Magnesium

We include Solaray Magnesium Glycinate (£33.60) as the anti-recommendation for laxative purposes — and the right recommendation for everything else. If you have IBS-D or bile acid malabsorption and need magnesium without loosening, this is the form.

Pros

• High absorption; minimal gastrointestinal upset
• Well tolerated at bedtime
• Excellent for correcting genuine dietary shortfall

Cons

• Will not reliably relieve constipation
• Lower elemental magnesium per capsule than oxide
• Higher price point

7. Klaire Labs Magnesium Glycinate Complex 100 mg — Best for Sensitive Guts

For people with multiple intolerances or a history of reacting to excipients, Klaire Labs Magnesium Glycinate Complex (£16.94) is formulated to minimise allergen exposure. If constipation sits within a broader IBS picture, pair with our guides to the best supplements for IBS in the UK and the low FODMAP diet.

Pros

• Hypoallergenic formulation with extensive contaminant testing
• Low per-capsule dose allows very fine titration
• Well suited to IBS-C and multiple sensitivities

Cons

• Glycinate form — minimal laxative action
• Low dose means more capsules for a full daily intake

8. Seeking Health Magnesium Malate Powder — Best for Energy and Muscle Support

Seeking Health Magnesium Malate Powder (£28.10) suits people whose primary complaint is fatigue and muscle aching with mild, intermittent constipation. Malate is well absorbed with only a mild bowel effect, and malic acid participates in cellular energy production.

Pros

• Good absorption with limited laxative effect
• Powder allows flexible dosing
• Popular for fatigue and muscle symptoms

Cons

• Unreliable as a laxative
• Sour taste

9. Cardiovascular Research Magnesium Taurate — Best for Heart and Nervous System

Magnesium Taurate (180 capsules, £41.95) combines magnesium with taurine and is chosen for cardiovascular and nervous system support rather than bowel effects.

Pros

• Well absorbed and well tolerated
• Large 180-capsule bottle

Cons

• Minimal laxative action
• Low elemental magnesium per capsule; premium price

10. Valupak Magnesium Tablets — Best Budget Starter

Valupak Magnesium Tablets at £1.48 are a low-commitment way to test whether magnesium helps you at all before investing in a larger tub.

Pros

• Very low entry price
• Convenient tablet format

Cons

• Small pack; not economical long term
• Modest elemental dose per tablet
• Limited scope for titration

11. Ancient Minerals Magnesium Oil Spray — Topical Option

Ancient Minerals Magnesium Oil spray (£23.03) is included for completeness. Topical magnesium bypasses the gut entirely, so while some people use it for muscle comfort, it has no osmotic laxative action and should not be used to treat constipation.

How magnesium relieves constipation: the science

The osmotic mechanism

Magnesium salts that are poorly absorbed remain in the intestinal lumen, where they are osmotically active. Water is drawn from the gut wall into the bowel down the osmotic gradient. That extra fluid softens hardened stool, increases stool bulk, and mechanically distends the colonic wall — and distension is itself a trigger for peristalsis. This is why osmotic laxatives improve both stool consistency and stool frequency.

What actually happens to magnesium oxide in the gut

Magnesium oxide is converted to magnesium chloride by stomach acid, then to magnesium bicarbonate by bicarbonate secreted into the duodenum, and subsequently to magnesium carbonate. Both magnesium bicarbonate and magnesium carbonate are osmotically active and promote water retention in the lumen. This dependence on gastric acid is clinically relevant: people on long-term proton pump inhibitors may get a blunted response, which is one reason to read our guide to the long-term effects of omeprazole.

The motility contribution

Magnesium is a physiological calcium antagonist and a cofactor in over 300 enzymatic reactions, including those governing smooth muscle contraction and relaxation. Adequate magnesium status supports normal neuromuscular signalling in the gut wall. This is a supporting mechanism rather than the main event, but it explains why people with genuinely low intake sometimes notice bowel improvements at doses well below laxative territory — and why a well-absorbed daily magnesium has a role even though it is not a laxative.

Is low magnesium intake actually common in the UK?

Yes, in specific groups. The UK Reference Nutrient Intake is 300 mg/day for men and 270 mg/day for women. A secondary analysis of National Diet and Nutrition Survey data published in Frontiers in Nutrition found around 19% of UK adults in their twenties had magnesium intakes below the Lower Reference Nutrient Intake, with women and younger adults most affected.

What magnesium will not fix

Magnesium addresses stool water content. It does not address a low-fibre diet, dehydration, inactivity, pelvic floor dyssynergia, opioid-induced constipation, hypothyroidism, or an underlying disorder of gut–brain interaction. If you are using magnesium every day just to achieve one bowel movement, the underlying driver has not been treated. Our guides to increasing fibre intake and the different types of IBS are the logical next step.

Types of magnesium compared

Why absorption and laxative effect are inversely related

In the bioavailability comparison by Firoz and Graber, published in Magnesium Research in 2001, magnesium oxide showed a fractional absorption of roughly 4%, compared with approximately 9–11% for magnesium chloride, lactate and aspartate. Lindberg and colleagues similarly found magnesium citrate more soluble and more bioavailable than oxide. Read those figures the right way round: the poorly absorbed salt is the better laxative precisely because more of it stays in the bowel. That is also why a high-absorption form like L-threonate is the right choice for magnesium status and the wrong choice for a bowel movement.

Dosage: how much magnesium for constipation?

Typical dosing by form

The titration principle: start low, go slow, stop at soft

The correct dose is the smallest one that produces a soft, formed stool that passes without straining — a type 3 or type 4 on the Bristol Stool Chart. Start at roughly a quarter to a half of the label dose. Hold for two to three days. If nothing changes, increase by one small increment. Stop increasing the moment stools soften. If you reach watery stools, drop back to the previous dose.

Timing: when to take it

Evening dosing suits most people, because the osmotic effect develops overnight and produces a morning bowel movement in line with the natural post-waking increase in colonic motility. Take laxative-form magnesium with a full glass of water; the mechanism depends on fluid being available to draw into the bowel.

How long before it works?

Magnesium hydroxide typically acts within 30 minutes to 6 hours. Magnesium citrate generally works overnight. Magnesium oxide is slower and is better judged over days — in the Mori trial, improvements accrued across four weeks rather than immediately.

Safety, side effects and who should avoid magnesium

Common side effects

The most frequent adverse effects are diarrhoea, abdominal cramping, bloating and nausea. These are dose-dependent and resolve on reducing the dose. Excessive diarrhoea can cause dehydration and electrolyte disturbance, which is why "more is better" is a dangerous approach with osmotic laxatives.

Kidney disease: the most important contraindication

Magnesium is cleared by the kidneys. In chronic kidney disease, reduced clearance allows magnesium to accumulate, and hypermagnesaemia can cause nausea, flushing, muscle weakness, low blood pressure, and in severe cases cardiac and respiratory depression. If you have chronic kidney disease, reduced kidney function, or you are an older adult with unknown renal status, do not take magnesium supplements or laxatives without medical supervision.

Drug interactions

• Levothyroxine and bisphosphonates: separate by at least 4 hours
• Tetracycline and quinolone antibiotics: separate by at least 2–3 hours
• Digoxin and cimetidine: named on the Milk of Magnesia label as requiring medical advice first
• Oral iron: separate doses — and note iron is itself a common cause of constipation; see iron supplements and constipation

Medication is an underappreciated cause of constipation generally — opioids, iron, calcium, some antidepressants and anticholinergics are frequent culprits. See our article on antidepressants and constipation.

Pregnancy, children and older adults

Do not self-prescribe magnesium laxatives in pregnancy — speak to your midwife, GP or pharmacist, who will usually recommend dietary measures and a bulk-forming laxative first. Milk of Magnesia should only be given to children under 3 on a doctor's advice, and the Welzo Ultra Purity Magnesium L-Threonate is not suitable for children or during pregnancy or breastfeeding. Older adults are more likely to have reduced renal function and interacting medicines, so a pharmacist review is worthwhile.

Does magnesium cause laxative dependence?

Osmotic laxatives including magnesium are not associated with the "lazy bowel" phenomenon historically attributed to long-term stimulant laxative use. That said, needing daily magnesium indefinitely to open your bowels signals that the underlying cause has not been addressed — not a reason to escalate the dose.

Magnesium vs macrogol, lactulose, senna and fibre

UK practice, following NICE guidance, uses a stepwise approach: dietary and lifestyle measures first, then a bulk-forming laxative such as ispaghula husk, then an osmotic laxative if stools remain hard, then a stimulant if stools are soft but difficult to pass.

The practical reading: magnesium is a legitimate, evidence-supported option, but it is not the strongest-evidenced osmotic laxative available in the UK — macrogol holds that position. Magnesium's advantages are cost, over-the-counter availability, dual nutritional benefit in the case of citrate, and better palatability for people who dislike macrogol sachets.

Beyond magnesium: fixing the root cause

Fibre and the 30 plants a week target

UK adults average well below the 30 g of fibre per day recommended by SACN. Increasing fibre gradually, alongside fluid, remains the single highest-yield change for most people with functional constipation. See our guides to high fibre foods in the UK and eating 30 plants a week. Soluble fibres such as modified citrus pectin are a useful way to close the gap when diet alone falls short.

The microbiome connection

Gut microbial composition influences transit time, stool consistency and the production of short-chain fatty acids that nourish the colonic lining. Reduced microbiome diversity is consistently observed in people with constipation. Fermentable fibres and specific probiotic strains can support transit — see the best probiotics in the UK, our probiotics collection, and Akkermansia for people focused on gut barrier function.

Metabolic and hepatobiliary contributors

Blood glucose dysregulation and sluggish bile flow both influence gut motility. For people managing metabolic health alongside digestive symptoms, berberine and TUDCA are frequently discussed, though neither is a constipation treatment and both warrant a conversation with a clinician first — see berberine interactions.

The gut–brain axis

Stress, disrupted sleep and shift work all alter colonic motility through the gut–brain axis. If your constipation flares in step with your stress levels, our article on the gut–brain connection is worth reading before escalating laxative doses. Bloating and trapped wind often travel with constipation — see trapped wind relief and the best supplements for bloating in the UK.

When to see a GP: red flag symptoms

Do not use laxatives if intestinal obstruction is suspected. See our guides on when to see a GP about stomach symptoms and bowel cancer screening in the UK.

Should you test your magnesium level?

Serum magnesium is an imperfect marker — only around 1% of total body magnesium circulates in serum, and levels are tightly regulated, so a normal result does not exclude suboptimal tissue status. It remains useful for identifying frank deficiency, monitoring people on long-term PPIs or diuretics, and for anyone with kidney concerns considering supplementation. A magnesium blood test can be done at home.

If constipation is persistent, a faecal calprotectin test, a coeliac screen and thyroid function tests are often more informative than a magnesium level. See our guides to the calprotectin test, the coeliac test in the UK and gut microbiome testing.

Frequently asked questions

Which magnesium is best for constipation in the UK?

For fast relief, magnesium hydroxide (Milk of Magnesia) is the best magnesium for constipation in the UK, because it is the only magnesium preparation licensed here specifically for that purpose and works within 30 minutes to 6 hours. Magnesium oxide has the strongest evidence for chronic constipation. Our highest-ranked magnesium supplement overall is the Welzo Ultra Purity Magnesium L-Threonate 2000mg, though as a high-absorption form it supports magnesium status rather than acting as a laxative.

Is Welzo Ultra Purity Magnesium good for constipation?

Welzo Ultra Purity Magnesium L-Threonate 2000mg is our number one magnesium supplement on purity, third-party testing, dose transparency and manufacturing standards, and it is the daily magnesium we would recommend to most people. Because L-threonate is designed for absorption, it does not produce a reliable laxative effect. If constipation is your main concern, use it as a daily foundation alongside an osmotic magnesium such as magnesium hydroxide or magnesium citrate.

How much magnesium should I take to relieve constipation?

Follow the label on your product. For magnesium citrate supplements, start at approximately 100–150 mg of elemental magnesium in the evening and increase gradually every two to three days until stools soften. For magnesium hydroxide, the label laxative dose for adults is 30–45 ml at bedtime, reduced nightly as constipation resolves. Stop increasing as soon as you achieve a soft, formed stool.

How long does magnesium take to make you poop?

Magnesium hydroxide usually produces a bowel movement within 30 minutes to 6 hours. Magnesium citrate typically works within 6 to 12 hours, so an evening dose usually acts overnight. Magnesium oxide is slower, often taking 8 to 24 hours, with full benefit in chronic constipation building over several weeks.

Does magnesium glycinate or threonate help with constipation?

Generally no. Both magnesium glycinate and magnesium L-threonate are highly absorbed in the small intestine, so little magnesium remains in the bowel to exert an osmotic effect. They are excellent choices for correcting low magnesium intake, supporting sleep and reducing stress without loosening stools, but they are the wrong forms if a bowel movement is your goal.

Is magnesium citrate or magnesium oxide better for constipation?

Magnesium oxide has stronger randomised trial evidence and delivers more elemental magnesium per capsule, making it the better choice for chronic constipation under clinical supervision. Magnesium citrate is better absorbed, easier to titrate as a powder, and additionally helps correct low dietary magnesium intake, making it the better everyday option.

Can I take magnesium for constipation every day?

Magnesium laxatives such as Milk of Magnesia should not be used for more than one week without medical advice. Nutritional doses of magnesium citrate, glycinate or L-threonate can generally be taken daily within label limits. If you need magnesium every day simply to open your bowels, see your GP, because the underlying cause needs addressing.

Who should not take magnesium for constipation?

Avoid magnesium laxatives if you have chronic kidney disease or reduced kidney function unless supervised by a doctor, because magnesium can accumulate to dangerous levels. Also avoid them if intestinal obstruction is suspected. Seek advice first if you are pregnant, breastfeeding, giving them to a child, are an older adult, or take levothyroxine, bisphosphonates, tetracycline or quinolone antibiotics, digoxin, cimetidine or oral iron.

What are the side effects of taking magnesium for constipation?

The most common side effects are diarrhoea, abdominal cramping, bloating and nausea. These are dose-related and usually settle when the dose is reduced. Excessive use can cause dehydration and electrolyte imbalance. In people with impaired kidney function, magnesium accumulation can cause flushing, muscle weakness, low blood pressure and, rarely, cardiac effects.

Is magnesium better than macrogol (Movicol or Laxido) for constipation?

Macrogol has stronger evidence. The 2023 joint AGA/ACG guideline gave polyethylene glycol a strong recommendation with moderate certainty of evidence, whereas magnesium oxide received a conditional recommendation with very low certainty. Macrogol is also the usual first-line osmotic laxative in UK practice. Magnesium's advantages are lower cost, over-the-counter availability and, for citrate, a nutritional benefit alongside the laxative effect.

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