How Long Does Miralax Take to Work? A Pharmacist’s Answers to This and 6 More Miralax FAQs

If you’ve ever been constipated, it’s likely that finding fast relief was a top priority. Often, people turn to over-the-counter (OTC) laxatives for help. But some laxatives can cause cramping and make you feel even more uncomfortable.

Miralax (polyethylene glycol 3350) is a popular option that can provide constipation relief without too much discomfort. But how long does Miralax take to work? And how often can you take it? Below, we’ll answer these and other common questions about Miralax to help you get the most from the medication. 

1. How long does Miralax take to work?

Miralax usually takes about 1 to 3 days to work. It’s an osmotic laxative, which works by pulling liquid into your stool. This helps make your stool softer and easier to pass. So drinking plenty of fluids while taking Miralax will help it work better.

Other laxatives, such as glycerin suppositories or senna tablets, work faster than Miralax. But they also typically have more risks and side effects to consider.

2. When does Miralax stop working?

The effects of Miralax usually only last for a day or so. It works as it travels through your digestive system. So Miralax’s effects gradually wear off as it passes through and out of your body with your stool. That’s why you need to take it every day that you’re constipated for it to continue working.

If you have loose stools for more than a day or two after stopping Miralax, let your healthcare team know. You should also seek medical care if your constipation returns while you’re still taking Miralax. In rare cases, Miralax may stop working if you develop a blockage in your bowel — more on that later. 

3. How much Miralax can you take?

The recommended Miralax dosage is 17 g of powder once a day until you have a bowel movement. It comes in individual premeasured packets or a larger container. If you have a container of Miralax, you’ll need to measure your dose. It’s usually equivalent to one capful (using the lid of the container). 

Each 17 g dose of Miralax should be mixed with 4 oz to 8 oz of liquid. Almost any liquid at any temperature is fine (such as water, juice, or coffee). You can also use carbonated drinks, but keep in mind that the bubbles may make you feel more bloated. Simply stir the powder into your chosen liquid until it’s dissolved, then drink it right away. 

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It’s OK to take Miralax daily for up to 1 week (7 days) at a time without a healthcare professional’s guidance. But after the first week, talk to your healthcare team if it isn’t working or you can’t move your bowels without it. There could be something more serious causing your symptoms.

Can you take Miralax more than once a day?

Don’t take Miralax more than once a day unless your prescriber specifically tells you to do so. You also shouldn’t take more than 1 dose at a time. Taking more Miralax than recommended won’t make it work faster. But it could increase the risk of side effects, such as diarrhea and dehydration.

That being said, your prescriber may recommend Miralax as a medication to prepare your bowels for a colonoscopy. In this case, they’ll give you specific instructions to follow. But you shouldn’t try to use Miralax this way on your own.

4. Can I take Miralax with my other medications?

Probably. Miralax isn’t known to interact with most medications. But it’s still a good idea to keep an up-to-date list of all medications and supplements you take and have a pharmacist or your prescriber review it before taking Miralax to be sure. 

5. Should you avoid certain foods while taking Miralax?

To avoid constipation in general, eating a fiber-rich diet helps. That’s because fiber can help move things through your gut. Oats, beans, and berries are all examples of high-fiber foods.

But too much fiber can cause or worsen constipation. So it may help to take a break from high-fiber foods while you’re constipated. Other foods to avoid that can worsen constipation include:

• Fried foods

• Processed meats

• Baked goods

• Dairy products

EXPERT PICKS: WHAT TO READ NEXT

• Miralax side effects: Learn about common and rare Miralax side effects, as well as how to manage them.

• Miralax vs. Colace: Here’s how these two OTC constipation products compare.

• Medications that cause constipation: Constipation has many possible causes. Could your medication be to blame? Read about the most common types of medications that cause constipation to find out.

6. What are the side effects of Miralax?

In most cases, Miralax is well-tolerated and causes fewer side effects than faster-acting OTC laxatives

The most common side effects of Miralax are:

• Bloating

• Gas

• Nausea

• Cramping 

But in rare cases, more serious risks and side effects are also possible. These are usually the result of something else going on in your body. Some examples include:

• Severe diarrhea: Miralax can sometimes cause diarrhea, especially in older adults. Diarrhea can lead to dehydration and other serious problems. If you develop diarrhea after taking Miralax, be sure to let your healthcare team know. 

• Bowel obstruction: A blockage in your intestine can be a serious problem. When it happens, food cannot move through your system properly. Contact a healthcare professional right away if you have symptoms of a bowel obstruction, such as severe pain in your abdomen along with bloating and nausea. If you don’t have a bowel movement within 1 to 3 days of taking Miralax, this may also be a sign of a blockage. 

• Allergic reactions: As with any medication, it’s possible to have an allergy to Miralax. Stop taking it and seek medical care if you have any signs of an allergy, including a rash, hives, or itching. A more serious allergic reaction (anaphylaxis) can occur, too. If you experience vomiting, difficulty breathing, or swelling in your face, call 911 or head to the nearest ER. 

7. Who shouldn’t take Miralax?

Even though Miralax is considered a safe medication, it may not be the best choice for everyone. This includes people with the following health conditions:

• History of bowel obstruction

• Appendicitis

• Inflammatory bowel disease

• Hole in the bowel (perforated bowel)

• Previous allergic reaction or known allergy to any ingredient in Miralax

People with kidney disease should also be cautious when taking Miralax. Talk to your prescriber about the risks and benefits of Miralax if you’re living with kidney problems.

Good to know: Miralax is considered safe to take during pregnancy, and it’s probably safe to take while breastfeeding, too. But it’s always a good idea to double-check with your prenatal care team to be sure it’s a safe option for you.

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Written by:

Stacia Woodcock, PharmD

Stacia Woodcock, PharmD, is a pharmacy editor for GoodRx. She earned her Doctor of Pharmacy degree from the University of Kentucky and is licensed in New York and Massachusetts.

Reviewed by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

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