Alginate vs. Tums for Heartburn: Calcium, Drug Interactions & Sodium

Most heartburn products look alike on the shelf. But two of the most common types work in very different ways. Tums-style tablets use calcium carbonate to cancel out stomach acid. Alginate products, like ØBEX, form a soft gel raft that floats on top of your stomach contents and blocks acid from coming back up.

For most people with occasional heartburn, either one is a reasonable choice. The differences matter more if you take other medicines, have kidney problems, are pregnant, or watch your sodium. This article covers what the research says on each point, with sources you can check yourself.

The short answer

Calcium carbonate neutralizes acid. It works within minutes. On an empty stomach, the effect lasts about 20 to 40 minutes (Salisbury & Terrell, 2023).
An alginate raft blocks reflux. It settles on the pool of acid that forms at the top of the stomach after a meal and acts as a barrier between that acid and the esophagus (Rohof et al., 2013).
Calcium carbonate can bind to some medicines and lower how much of them you absorb. Thyroid medicine and some antibiotics are the best-known examples (Ogawa & Echizen, 2011).
Too much calcium carbonate can raise blood calcium. This is rare at label doses, but it's a known risk with heavy use (Patel & Goldfarb, 2010).
ØBEX contains no calcium, magnesium, or aluminum. So it doesn't interact with medicines the way calcium does. The only thing to watch is physical: take other medicines 10 to 15 minutes before ØBEX so they don't get trapped in the raft.
Each 5 mL dose of ØBEX has 65 mg of sodium, or 3% of the daily value. Four doses a day is 260 mg. For most people that's small. If you're on a sodium-restricted diet, count it.

How calcium carbonate (Tums) works

Calcium carbonate is a base. When it meets stomach acid, the two cancel each other out and the acid gets weaker. That's why relief can be fast.

The catch is timing. On an empty stomach, an antacid leaves the stomach quickly, so the effect lasts about 20 to 40 minutes. Taken about an hour after a meal, the food slows things down and the effect can last about three hours (Salisbury & Terrell, 2023).

Calcium carbonate is also about 40% calcium by weight. So a regular-strength 500 mg tablet carries about 200 mg of calcium. Some of that calcium is absorbed into your blood. For most people, that's harmless. For some, it adds up. More on that below.

You may have heard that calcium carbonate causes "acid rebound," where your stomach makes extra acid once the tablet wears off. Lab studies do show that calcium can stimulate acid. But a review of the evidence found that rebound was mainly a lab finding, with no clear harm shown for people using antacids (Hade & Spiro, 1992). So it's not a strong reason to avoid calcium carbonate.

How an alginate raft works

Alginate is a natural fibre from brown seaweed. In ØBEX, sodium alginate is paired with a small amount of sodium bicarbonate. When the mix meets stomach acid, the alginate turns into a gel. The bicarbonate releases carbon dioxide bubbles that get trapped in the gel, so the gel floats (Mandel et al., 2000).

After a meal, fresh acid collects in a layer at the top of the stomach, close to the valve that leads to the esophagus. Researchers call this layer the acid pocket. In one imaging study, an alginate raft settled right on top of the acid pocket and stayed there for the full two hours of measurement. People who took it had a median of 3.5 acid reflux episodes, compared with 15 for people who took a plain antacid (Rohof et al., 2013). We cover that study in How an Alginate Raft Stops Reflux: The Acid Pocket on Camera.

Here's the key difference. An antacid changes the acid. A raft changes where the acid can go.

The alginate itself isn't absorbed. Your body treats it like dietary fibre, and it leaves the body the way fibre does. We explain that journey in What Happens to the Alginate Raft After It Does Its Job?

Which works better: alginate or antacid?

Here's what the stronger studies show.

A meta-analysis pooled 14 randomized trials with 2,095 people. Alginate products were about four times more likely than placebo or antacids to resolve reflux symptoms (odds ratio 4.42). One caveat: that number combines the placebo and antacid comparisons (Leiman et al., 2017).
A double-blind crossover trial gave people with reflux either an alginate-antacid liquid or a plain antacid with the same acid-neutralizing strength after a meal. With the alginate product, the esophagus was exposed to less acid after the meal. The number of reflux events was about the same, but the reflux that did happen was less acidic (de Ruigh et al., 2014).
A mechanism study found that after a fatty meal, 8 of 10 people with reflux had a layer of acid reaching the valve. An alginate-antacid liquid cleared it in 6 of those 8 and pushed the acid layer away from the valve (Kwiatek et al., 2011).
Specialist guidance from the American Gastroenterological Association lists alginate antacids as an add-on for breakthrough symptoms. It says they may help most with after-meal or night-time symptoms, and in people with a hiatal hernia (Yadlapati et al., 2022).

And the honest limits:

Several of these studies tested alginate products that also contain calcium carbonate. So they show the value of adding a raft. They don't show that calcium carbonate is useless.
Many alginate studies were small, and several were funded by a company that makes alginate products.
None of these studies tested ØBEX. They tested alginate as an ingredient.

For more on the evidence, see our research page and our review of adding an alginate to a PPI for night-time heartburn.

Why calcium carbonate can be a problem for some people

Calcium carbonate is safe for most people at the doses on the label. Problems show up with heavy or long-term use, or in people whose bodies handle calcium differently.

Calcium-alkali syndrome

Taking in a lot of calcium along with an absorbable base can cause calcium-alkali syndrome. It used to be called milk-alkali syndrome. It has three parts: high blood calcium, too much alkali in the blood, and kidney injury. Calcium carbonate supplies both the calcium and the alkali.

Cases have risen along with the use of over-the-counter calcium. It's now the third most common reason people are admitted to hospital with high blood calcium (Patel & Goldfarb, 2010). If you take antacids every day and feel unwell, tell your doctor exactly what you're taking and how much.

Pregnancy

Heartburn is common in pregnancy, and calcium carbonate antacids are widely used. In 2024, Health Canada reviewed reports of calcium-alkali syndrome in pregnancy linked to these antacids. It found 2 Canadian adverse reaction reports and 8 published case reports. In every case, the person had taken more than the label allows. Pregnancy hormones increase how much calcium the gut absorbs, which likely raises the risk (Health Canada, 2024).

The message isn't "never use calcium carbonate while pregnant." It's "stick to the label, and count every source of calcium, including prenatal vitamins." Alginate products have also been studied in pregnancy, in open-label studies with no comparison group (Lindow et al., 2003; Strugala et al., 2012). Our page on heartburn relief during pregnancy has more. Always check with your prenatal care provider.

Kidney disease and kidney stones

Your kidneys clear extra minerals from your blood. When they don't work well, minerals like calcium and sodium can build up. BC Renal lists calcium carbonate as an option people with kidney disease can use as needed, and advises talking to a doctor or pharmacist before taking anything regularly (BC Renal, 2023). If you've had kidney stones, ask your doctor before using calcium antacids often. Health Canada's upper limits for calcium are set partly to protect against stones (Health Canada, n.d.-d).

It adds up

Health Canada sets an upper limit for total calcium from all sources. It's 2,500 mg a day for adults aged 19 to 50, and 2,000 mg a day for adults 51 and older (Health Canada, n.d.-d). Food, supplements, and antacids all count. Ten regular-strength tablets carry about 2,000 mg of calcium on their own (Health Canada, 2024). If you also take a calcium supplement or a prenatal vitamin, it's easy to go over without noticing.

Which medications interact with calcium carbonate?

Calcium carbonate can interfere with other medicines in two main ways (Ogawa & Echizen, 2011):

Binding. Calcium can latch onto some drugs in the gut and form a compound your body can't absorb well.
Changing stomach acidity. Some drugs need acid to dissolve. Less acid can mean less of the drug gets absorbed.

Examples from the research:

Thyroid medicine (levothyroxine). In 20 people with an underactive thyroid, taking calcium carbonate with levothyroxine for three months lowered thyroid hormone levels. TSH, the marker doctors use to adjust the dose, rose from 1.6 to 2.7 mIU/L. It went back down after the calcium was stopped (Singh et al., 2000).
Ciprofloxacin (an antibiotic). Calcium carbonate taken just before ciprofloxacin cut its absorption to about 60% of normal (Frost et al., 1992).
Tetracycline antibiotics and bisphosphonates (bone medicines like alendronate) can also be bound by calcium (Ogawa & Echizen, 2011).
Dolutegravir (an HIV medicine). Taken with calcium carbonate on an empty stomach, drug levels dropped by about 39%. Taking it with a meal, or 2 hours before the calcium, avoided the drop (Song et al., 2015).

That's why Health Canada's labelling standard for antacids tells people not to take an antacid within 2 hours of another medicine, because it may change how well that medicine works (Health Canada, n.d.-a).

Does ØBEX interact with medications?

This is the question we hear most. The short answer: no, not the way calcium carbonate does. The one thing to plan for is physical, not chemical. A pill taken at the same moment as ØBEX can get caught in the raft as it forms. A 10 to 15 minute gap solves that.

Why ØBEX doesn't interact like calcium carbonate

No calcium, magnesium, or aluminum. These minerals are behind the best-known antacid–drug interactions, because they bind to drugs like thyroid medicine and some antibiotics (Ogawa & Echizen, 2011). The active blend in ØBEX is sodium alginate and sodium bicarbonate. Sodium doesn't bind to drugs the way calcium does.

The alginate isn't absorbed. It stays in your gut and leaves as fibre. It doesn't enter your bloodstream, so it doesn't compete with other drugs there.

It barely changes stomach acidity. You can check this from our product label. Each 5 mL dose has 65 mg of sodium. Sodium bicarbonate is about 27% sodium by weight. So even if all of that sodium came from bicarbonate, a dose could hold at most about 240 mg of it. (It holds less, since sodium alginate carries sodium too.) That much bicarbonate can neutralize at most about 3 milliequivalents (mEq) of acid. A single 500 mg calcium carbonate tablet can neutralize about 10 mEq. So one dose of ØBEX has less than a third of the acid-neutralizing power of one regular-strength tablet. The bicarbonate is there mainly to make the bubbles that float the raft.

One study found no effect on a common reflux drug. In 26 volunteers, taking a liquid alginate with omeprazole didn't change how much omeprazole the body absorbed (Dettmar et al., 2006).

The one thing to watch: trapping

The raft is a gel. Anything swallowed at the same moment can get caught inside it as it forms. Research shows why that matters: in a small trial, iron sealed inside alginate beads was absorbed less than iron from a capsule, 8.5% versus 12.6% (Wawer et al., 2014). That's a trapping effect, not a chemical reaction, and it's easy to avoid.

The simple rule

Take your other medicines first, then wait 10 to 15 minutes before taking ØBEX. That gives pills and liquids time to move past the top of the stomach before the raft forms. If you have questions about your own medicine list, your pharmacist can help.

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Alginates are also commonly used alongside acid-reducing pills. In trials, they've been added on top of a daily PPI, and specialist guidance lists them as an add-on for breakthrough symptoms (Yadlapati et al., 2022).

How much sodium is in ØBEX, and does it matter?

Each 5 mL (one teaspoon) dose of ØBEX has 65 mg of sodium. That's 3% of the 2,300 mg daily value used on Canadian labels (Health Canada, n.d.-b). If you follow the label and take it after three meals and before bed, four doses a day add up to 260 mg, or about 11% of the daily value.

The sodium comes from both active ingredients: sodium alginate and sodium bicarbonate. In the stomach, the alginate gives up its sodium as the gel forms, and that sodium is absorbed like any other. So it counts toward your daily total.

For perspective:

Canadians eat about 2,760 mg of sodium a day on average (Health Canada, 2018). Four doses of ØBEX is less than a tenth of that.
Health Canada says 5% of the daily value or less is "a little," and 15% or more is "a lot" (Health Canada, n.d.-c). One dose is a little. A full day of doses falls in between.
Other alginate liquids are in the same range. The UK's Gaviscon Advance lists 57.85 mg of sodium per 5 mL. Its label asks people on a low-salt diet, such as some people with heart failure or kidney problems, to take that into account (Reckitt Benckiser Healthcare, n.d.).

Who should pay attention

Most people won't notice 65 to 260 mg of sodium a day. It matters if your doctor has given you a sodium limit. That's common with:

heart failure
chronic kidney disease
high blood pressure that's hard to control

In the US, any over-the-counter oral medicine with more than 140 mg of sodium in its maximum daily dose must carry the warning "Ask a doctor before use if you have a sodium-restricted diet" (Sodium Labeling, 2026). ØBEX's 260 mg a day is above that line. So we give the same advice: if you're on a sodium-restricted diet, check with your doctor or pharmacist first.

Sodium from medicines is easy to forget. A large UK study linked sodium-heavy dissolving and fizzy medicines, many of which carry far more sodium per dose than ØBEX, to a higher risk of heart attack, stroke, or vascular death (odds ratio 1.16). It also found much higher odds of high blood pressure (George et al., 2013). The takeaway: sodium in medicine counts, so include it in your daily total.

The honest trade-off

Calcium carbonate itself contains no sodium. So if sodium is your main concern, and calcium isn't, a calcium antacid used now and then may suit you better. Some alginate products cut sodium by using potassium bicarbonate instead, but that adds potassium, which people with kidney disease may also need to limit.

There's no perfect choice for everyone. BC Renal, for example, notes that antacids with sodium can raise blood pressure in people with kidney disease, and lists calcium carbonate as an as-needed option (BC Renal, 2023). If you have kidney disease, let your kidney team help you choose.

Alginate or calcium carbonate: which fits you?

This isn't advice for your specific case. But here's how the evidence lines up.

Occasional heartburn, no other health issues: Either can work. Calcium carbonate is quick and cheap. An alginate raft is built for after-meal and lying-down reflux.
You take thyroid medicine, certain antibiotics, bone medicines, iron, or HIV medicine: Calcium carbonate needs careful timing. ØBEX is simpler: just take your medicines 10 to 15 minutes before it.
You already take a calcium supplement or prenatal vitamin: Watch your total calcium. ØBEX adds none.
You have kidney disease or have had kidney stones: Both calcium and sodium matter. Talk to your kidney team before using any antacid regularly.
You're on a sodium-restricted diet: Count ØBEX's sodium (65 mg per dose) and check with your care team.
You're pregnant: Both types are used in pregnancy. Follow the label and ask your prenatal care provider.
You take a PPI and still have symptoms, especially at night: Alginates have been studied as an add-on. See our night-time heartburn article.

When to see a doctor

Over-the-counter heartburn products are for occasional symptoms. See a doctor if:

you need heartburn relief more than twice a week, or for longer than two weeks
it hurts to swallow, or food feels stuck
you have unexplained weight loss, repeated vomiting, or black stools

If you have chest pain with shortness of breath, sweating, or pain spreading to your arm or jaw, call 911. It may not be heartburn. Our comprehensive guide to heartburn covers more warning signs.

The bottom line

Calcium carbonate and alginate rafts both relieve heartburn, but they work differently. Calcium carbonate neutralizes acid quickly and briefly, and it's safe for most people at label doses. Its calcium can bind to some medicines and can build up with heavy use, in pregnancy, or with kidney problems.

An alginate raft works by position, not by neutralizing. It floats on the acid pocket and acts as a barrier. ØBEX has no calcium, magnesium, or aluminum, and very little acid-neutralizing power, so it doesn't interact with medicines the way calcium does. Just take other medicines 10 to 15 minutes before ØBEX so they don't get trapped in the raft. And each dose has 65 mg of sodium, which is worth counting if you're on a sodium-restricted diet.

When in doubt, bring your medicine list to your pharmacist. They can help you pick the right option and the right timing.

References

BC Renal. (2023). Non-prescription medications and your kidneys [Patient handout]. Provincial Health Services Authority. https://www.bcrenal.ca/resource-gallery/Documents/Non-Prescription_Medications_and_Your_Kidneys.pdf

de Ruigh, A., Roman, S., Chen, J., Pandolfino, J. E., & Kahrilas, P. J. (2014). Gaviscon Double Action Liquid (antacid & alginate) is more effective than antacid in controlling post-prandial oesophageal acid exposure in GERD patients: A double-blind crossover study. Alimentary Pharmacology & Therapeutics, 40(5), 531–537. https://doi.org/10.1111/apt.12857 (PMID 25041141)

Dettmar, P. W., Hampson, F. C., Jain, A., Choubey, S., Little, S. L., & Baxter, T. (2006). Administration of an alginate based gastric reflux suppressant on the bioavailability of omeprazole. Indian Journal of Medical Research, 123(4), 517–524. (PMID 16783042)

Frost, R. W., Lasseter, K. C., Noe, A. J., Shamblen, E. C., & Lettieri, J. T. (1992). Effects of aluminum hydroxide and calcium carbonate antacids on the bioavailability of ciprofloxacin. Antimicrobial Agents and Chemotherapy, 36(4), 830–832. https://doi.org/10.1128/AAC.36.4.830 (PMID 1503446)

George, J., Majeed, W., Mackenzie, I. S., MacDonald, T. M., & Wei, L. (2013). Association between cardiovascular events and sodium-containing effervescent, dispersible, and soluble drugs: Nested case-control study. BMJ, 347, f6954. https://doi.org/10.1136/bmj.f6954 (PMID 24284017)

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Health Canada. (n.d.-d). Vitamin D and calcium: Updated dietary reference intakes. Retrieved September 26, 2026, from https://www.canada.ca/en/health-canada/services/food-nutrition/healthy-eating/vitamins-minerals/vitamin-calcium-updated-dietary-reference-intakes-nutrition.html

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Kwiatek, M. A., Roman, S., Fareeduddin, A., Pandolfino, J. E., & Kahrilas, P. J. (2011). An alginate-antacid formulation (Gaviscon Double Action Liquid) can eliminate or displace the postprandial 'acid pocket' in symptomatic GERD patients. Alimentary Pharmacology & Therapeutics, 34(1), 59–66. https://doi.org/10.1111/j.1365-2036.2011.04678.x (PMID 21535446)

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Strugala, V., Bassin, J., Swales, V. S., Lindow, S. W., Dettmar, P. W., & Thomas, E. C. M. (2012). Assessment of the safety and efficacy of a raft-forming alginate reflux suppressant (Liquid Gaviscon) for the treatment of heartburn during pregnancy. ISRN Obstetrics and Gynecology, 2012, Article 481870. https://doi.org/10.5402/2012/481870 (PMID 23209926)

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Related reading on the ØBEX blog:

This article is for general education and is not medical advice. It compares ingredient classes; the studies cited tested other alginate and antacid products, not ØBEX. Tums and Gaviscon are trademarks of their respective owners, and ØBEX is not affiliated with them. These statements have not been evaluated by Health Canada. Do not stop or change how you take any prescribed medication based on this article. Talk to your doctor or pharmacist if you take other medicines, are pregnant, have kidney or heart disease, are on a sodium-restricted diet, or have heartburn that keeps coming back.