Choosing the best magnesium for constipation isn’t as simple as grabbing whatever’s on the shelf. It’s one of the most common questions I get asked in clinic — and one of the most misunderstood.

In this episode, I’m walking you through the actual mechanism of how magnesium moves your bowels, what the research really shows about magnesium oxide, and why I still start almost every client on citrate or bisglycinate instead — especially if SIBO or IMO is part of your picture.

Whether you’ve been handed magnesium oxide at the chemist and it’s just not cutting it, or you’re constipated and trying to work out which form is actually worth taking — this one is for you.

It doesn’t have to be complicated. There’s a lot of conflicting advice out there about magnesium. But once you understand what’s actually happening in your gut, the choice becomes a lot clearer.

Why Not All Magnesium Is the Same

Magnesium works by pulling water into your gut. That softens the stool and gets things moving — think of it as nature’s stool softener.

Low magnesium intake is genuinely linked to a higher risk of constipation, and most of us simply don’t get enough from food alone.

But here’s the thing: not all magnesium behaves the same way in the body. And that difference is what this whole episode is about.

How Magnesium Oxide Actually Works

When you swallow magnesium oxide, your stomach acid breaks it down into magnesium chloride. Further along, your pancreas releases its own secretions, converting it again — this time into magnesium carbonate and magnesium bicarbonate. Those are the compounds that actually pull water into your colon, soften your stool, and speed things up.

Here’s the key part: oxide only works after this whole multi-step conversion process, and even then, only about 4% of it actually gets absorbed. The rest stays in your gut — which is exactly why it works as a laxative in the first place.

Does it actually work? Yes. One randomised, double-blind, placebo-controlled trial gave 34 women with mild-to-moderate constipation 1.5g of magnesium oxide a day for 28 days. Colon transit time nearly halved — dropping from over 75 hours to under 42 — and bowel movements went from less than two a week to just over six. In a separate study, magnesium oxide performed just as well as senna (a common herbal laxative) for chronic constipation.

So let’s be clear: oxide isn’t “bad.” It works, and there’s solid evidence behind it.

The Alkalising Effect — and the Methane Connection

Here’s the part I find really interesting. That same process that makes oxide effective also does something else — it makes your colon more alkaline.

Separately, we know that the organisms behind methane-driven constipation — that’s SIBO-methane, or IMO — aren’t even bacteria. They’re archaea. And they don’t like an acidic gut; they thrive when things are more alkaline. The bacteria that keep them in check, on the other hand, prefer a more acidic environment.

Put those two things together, and you get a pretty compelling theory: if you’ve got methane-driven constipation, taking an alkalising laxative like magnesium oxide might actually be feeding the very organisms slowing you down.

Important note: this hasn’t been proven in a clinical trial. It’s a biologically sound theory, not a proven fact — but it’s part of why I generally don’t use magnesium oxide, not just in these particular cases.

By contrast, magnesium citrate and bisglycinate don’t break down in the stomach the same way oxide does, so they’re less likely to have that same alkalising effect on the colon. This hasn’t been directly tested either, but the reasoning holds up.

The Best Magnesium for Constipation: Citrate, Then Bisglycinate

Citrate and bisglycinate are both far better absorbed than oxide. They’re gentler on the gut, and they come with extra benefits too — including support for your nervous system, which matters if stress is playing a role in your constipation.

My go-to clinical approach: start with magnesium citrate in the evening. Once bowel movements start to regulate, transition over to magnesium bisglycinate for even better absorption, plus that added nervous system support.

Most of the research on magnesium citrate uses around 300mg of elemental magnesium a day. If you’re concerned about how your body might respond, you can start on a lower dose and build up to that amount over a week or so. If you get loose stools along the way, that’s your sign to ease back slightly.

⚠️ Important note: If you’re on medications, have kidney issues, or are unsure if magnesium is appropriate for you, please check with your practitioner before supplementing.

The Recap

• Magnesium oxide: works via a multi-step conversion process, and the evidence for its effectiveness is solid — but it may alkalise the colon in a way that’s worth being cautious about, particularly with methane-driven constipation.
• Magnesium citrate: well absorbed, gentler on the gut, and my usual starting point — around 300mg of elemental magnesium daily, titrated up if needed.
• Magnesium bisglycinate: my preferred long-term option once bowel movements regulate, for its absorption and added nervous system support.
• Always: check with your practitioner if you’re on medications or have kidney issues before supplementing.

When Does Magnesium Stop Being Enough?

If your constipation actually gets worse after increasing magnesium, fibre, or fruit — that’s a red flag. Don’t push through it. That’s usually a sign there’s something else going on, and it’s worth getting properly investigated.

Magnesium is one tool, not the whole toolkit. Fluid, exercise, fibre, and actually establishing what’s driving your constipation in the first place all matter just as much. And if methane turns out to be part of your picture, you’ll often need more targeted strategies on top of that.

THIS EPISODE IS FOR YOU IF…

• You’ve been told to “just take magnesium,” and it’s not working
• You’ve been using magnesium oxide and want to understand what it’s actually doing in your body
• You have SIBO, IMO, or suspect methane might be part of your constipation picture
• You want a simple, evidence-based explanation — not a blanket “avoid this” rule
• You’re ready to understand what’s actually driving your constipation, rather than just masking it

Research referenced in this episode:

1. Murakami K, Sasaki S, Okubo H, Takahashi Y, Hosoi Y, Itabashi M. Association between dietary fiber, water and magnesium intake and functional constipation among young Japanese women. Eur J Clin Nutr. 2007;61(5):616-22.
2. Mori S, et al. A Randomized Double-blind Placebo-controlled Trial on the Effect of Magnesium Oxide in Patients With Chronic Constipation. J Neurogastroenterol Motil. 2019.
3. Morishita D, Tomita T, Mori S, Kimura T, Oshima T, Fukui H, Miwa H. Senna Versus Magnesium Oxide for the Treatment of Chronic Constipation: A Randomized, Placebo-Controlled Trial. Am J Gastroenterol. 2021;116(1):152-161.
4. Mori H, Tack J, Suzuki H. Magnesium Oxide in Constipation. Nutrients. 2021;13(2):421.
5. Omori K, Miyakawa H, Watanabe A, Nakayama Y, Lyu Y, Ichikawa N, Sasaki H, Shibata S. The Combined Effects of Magnesium Oxide and Inulin on Intestinal Microbiota and Cecal Short-Chain Fatty Acids. Nutrients. 2021;13(1):152.
6. Ishikado A, Sato T, Mitsuoka T. Suppressive effects of lactulose and magnesium oxide supplementation on fecal putrefactive metabolites with shortening gastrointestinal transit time. Microb Ecol Health Dis. 2007;19(3):184–190.
7. Abell GCJ, Conlon MA, McOrist AL. Methanogenic archaea in adult human faecal samples are inversely related to butyrate concentration. Microb Ecol Health Dis. 2006;18:154-160.
8. Lindberg JS, Zobitz MM, Poindexter JR, Pak CY. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990;9(1):48-55.
9. Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research. 2003;16(3):183-91.

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Connect with gut health naturopath Lynda Griparic: Instagram

As a naturopath with extensive experience in complex gut health conditions, I offer telehealth consultations across Australia and internationally. If you’ve been taking magnesium and it’s just not cutting it — or if it seems to be making things worse — self-prescribing can only take you so far. I would love to help you work out why. This is exactly the kind of work I do.

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If you have a gut health question you’d like answered or would like to discuss your situation, email info@lyndagriparic.com or leave a comment below.

This content is for educational purposes only and is not intended as medical advice. Please consult your healthcare provider before making any changes to your diet or health routine.