Certain antacids, particularly those containing aluminum or calcium, are well-known causes of constipation. The good news is that this side effect is manageable. You can often fix it by switching to a different type of antacid, adjusting your fiber and fluid intake, or using a short-term laxative while you sort out a longer-term plan.
Why Some Antacids Cause Constipation
Not all antacids have this effect. The ones that slow your bowels down are aluminum-based and calcium-based formulas. Aluminum hydroxide is the most common culprit. It slows intestinal movement and, with prolonged use, can even cause obstruction in severe cases. Calcium carbonate (the active ingredient in Tums and similar products) can also back things up, though usually less dramatically.
These minerals interact with your digestive tract in ways that reduce the water content of stool and slow its passage through the intestines. The longer stool sits in your colon, the more water gets absorbed from it, making it harder and more difficult to pass. If you’re taking these antacids multiple times a day or over several weeks, the effect compounds.
Switch to a Magnesium-Based Antacid
The simplest fix is switching to an antacid that contains magnesium hydroxide. Magnesium neutralizes stomach acid just like aluminum and calcium do, but it has the opposite effect on your bowels. It works as a saline laxative, drawing water into the intestines and keeping stool soft. This is why products like Milk of Magnesia are sold both as antacids and as laxatives.
Some antacid brands combine magnesium with aluminum specifically to balance out the constipating effect of aluminum with the loosening effect of magnesium. If you don’t want to switch entirely, look for a combination formula. Take it with a full glass of water (about 8 ounces) for best results.
Increase Fiber Gradually
Adding fiber to your diet is one of the most effective ways to counteract constipation from any cause, including antacids. Fiber adds bulk to stool and helps it move through your intestines more efficiently. The daily targets most people should aim for: at least 25 grams for women under 50 (21 grams for women over 50) and at least 38 grams for men under 50 (30 grams for men over 50). Most Americans get roughly half that amount.
Good sources include beans, lentils, oats, whole wheat bread, berries, pears, broccoli, and ground flaxseed. If you’re not used to eating much fiber, increase your intake slowly over a week or two. Adding too much at once can cause bloating and gas, which won’t help when you’re already dealing with digestive discomfort from the antacids.
Drink More Water Than You Think You Need
Fiber only works well if you’re drinking enough fluid alongside it. Research on adults with chronic constipation found that a daily fiber intake of 25 grams significantly increased bowel movement frequency, but the effect was even stronger when fluid intake reached 1.5 to 2 liters per day (roughly 50 to 67 ounces). People in the study who drank only about 1 liter daily saw less benefit from fiber alone.
This matters even more when you’re taking aluminum or calcium antacids, because those minerals pull water out of your stool. You need to replace that lost moisture. Aim for at least 8 glasses of water spread throughout the day, and more if you’re physically active or in a warm climate.
Try an Over-the-Counter Osmotic Laxative
If dietary changes aren’t enough, an osmotic laxative like polyethylene glycol (sold as MiraLax and several store brands) can help. It works by pulling water into your colon to soften stool, similar to how magnesium works but through a different mechanism. You dissolve the powder in 4 to 8 ounces of water, juice, coffee, or tea and drink it. Avoid mixing it with starch-based thickeners.
This type of laxative is generally well tolerated, but you shouldn’t use it for more than one week without checking with your doctor. It’s meant as a bridge while you make other changes, not a permanent solution layered on top of a constipating antacid.
Rethink Your Antacid Timing and Frequency
According to the NHS, antacids work best and last longest when taken with food or shortly after eating, which is when heartburn and indigestion are most likely to strike. Taking them at the right time means you may be able to use less overall, reducing the constipation effect.
If you’re reaching for antacids more than a few times a week, it’s worth considering whether a different type of acid-relief medication might work better for you. Antacids are designed for occasional, short-term use. Frequent heartburn that requires daily antacids often responds better to other approaches that don’t carry the same constipation risk.
Signs the Constipation Needs Medical Attention
Most antacid-related constipation resolves once you adjust your approach. But certain symptoms signal something more serious. Contact a doctor if you notice blood in your stool, rectal bleeding, constant abdominal pain, inability to pass gas, vomiting, fever, unexplained weight loss, or lower back pain alongside your constipation. You should also seek care if constipation doesn’t improve with the self-care strategies above, or if you have a family history of colon or rectal cancer.