Quick answer

Ask your pharmacist or doctor before combining. NIH says magnesium-rich supplements or medicines can decrease the absorption of oral bisphosphonates such as alendronate. Antacids and laxatives count: milk of magnesia carries 500 mg of magnesium per tablespoon. The alendronate label adds that mineral water, food and some other medicines are likely to reduce absorption too.^1,2

Key facts

NIH says magnesium-rich supplements or medications can decrease the absorption of oral bisphosphonates, such as alendronate.¹
NIH's consumer sheet says bisphosphonates are not well absorbed when taken too soon before or after supplements or medicines with high amounts of magnesium.³
The alendronate label says other beverages, including mineral water, food and some medicines are likely to reduce its absorption.²
MedlinePlus names milk of magnesia among the antacids to discuss with your doctor before you start alendronate.⁴
Over-the-counter magnesium oxide antacid labels carry a general warning that antacids may interact with certain prescription drugs.⁵

Ask your pharmacist how far apart to take them

Supplements and medicines rich in magnesium can reduce the absorption of oral bisphosphonates such as alendronate. The alendronate label says calcium, antacids and oral medicines containing minerals such as magnesium interfere with its absorption.^1,2,3

On the medicine's label

What does NIH say about magnesium and oral bisphosphonates?

NIH's health professional sheet on magnesium lists oral bisphosphonates, with tetracycline and quinolone antibiotics, as medicines magnesium may interact with. It uses alendronate as the example and says magnesium-rich supplements or medications can decrease absorption. The consumer sheet puts it plainly: bisphosphonates are not well absorbed when taken too soon before or after supplements or medicines with a lot of magnesium.^1,3

The health professional sheet also describes how far apart the two can be taken. That spacing is for your pharmacist to confirm against your own prescription, so it is not repeated here.¹

Neither source narrows the warning to one form of magnesium. The sources checked do not compare glycinate with citrate or oxide on this point. That is not a finding either way, so ask your pharmacist about the one you use.^1,3

Where does magnesium hide in products and drinks?

Some antacids and laxatives carry large amounts of magnesium: Milk of Magnesia provides 500 mg per tablespoon and Extra Strength Rolaids 55 mg per tablet, while Tums contains none.¹

Tap, mineral and bottled waters can supply magnesium, from 1 mg to more than 120 mg per liter depending on the source and brand.¹

Magnesium oxide is sold as an antacid, and its Drug Facts label says antacids may interact with certain prescription drugs. Magnesium citrate is sold as a supplement and as a liquid laxative. Because the alendronate label lists mineral water among the drinks that reduce absorption, the label on your water bottle is worth reading on the morning you take your tablet.^2,5,6

What does the alendronate label add?

The label explains the stakes. Taking alendronate with food, with drinks other than plain water or with other medicines lessens its effect by decreasing how much is absorbed. Its Medication Guide lists antacids among the medicines to tell your doctor about before you start.²

MedlinePlus says that if you take any other medicines, supplements, vitamins or antacids by mouth, they should wait until after the alendronate dose, and it names milk of magnesia among the antacids to raise with your healthcare provider first.⁴

Does the warning cover bisphosphonates given by injection?

NIH's wording is about oral bisphosphonates, the tablets and liquids you swallow. The sources checked do not address bisphosphonates given by injection or infusion. That is not a finding either way, so ask your pharmacist about yours.¹

What to ask your pharmacist about magnesium and bisphosphonates

Which of my products contain magnesium, including antacids, laxatives, magnesium supplements and multivitamins?
How do I fit magnesium around my bisphosphonate on dosing days?
Does the mineral water I drink matter on the morning I take my tablet?
Is my bisphosphonate one of the oral ones these sources cover?

Take the Supplement Facts or Drug Facts panel of each product with you. NIH says the panel lists the amount of the magnesium itself, not the weight of the whole compound.¹

Questions people ask

Can you take magnesium with bisphosphonates?

NIH and the alendronate label both say magnesium-containing products can interfere with absorption when they follow an oral bisphosphonate too closely. Ask your pharmacist how to arrange your own doses.^1,2

Does magnesium glycinate affect bisphosphonates?

The NIH sheets refer to magnesium-rich supplements in general and do not single out glycinate. The sources checked do not address it separately. That is not a finding either way, so ask your pharmacist.^1,3

Do magnesium supplements and bisphosphonates need to be kept apart?

NIH says taking them too close together reduces absorption of the bisphosphonate, and the label says to wait after a dose before any other oral medicine. Your pharmacist can give the spacing that fits your prescription.^2,3

Do magnesium antacids count as magnesium supplements here?

Yes, in the sense NIH describes: antacids and laxatives can carry a lot of magnesium, as much as 500 mg per tablespoon for milk of magnesia. The alendronate label names antacids too.^1,2

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This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.