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How long does it take for MiraLAX to work?

Here’s when to expect relief when taking this OTC laxative
Leandra Beabout
By | Nov. 29, 2023 Medically reviewed by Jesse P. Houghton, MD, FACG
A clock — how long does it take for MiraLAX to work

Key takeaways

  • MiraLAX, an over-the-counter osmotic laxative for occasional constipation, generally takes 12 to 24 hours to work but can take up to 96 hours in some cases.

  • Factors influencing how long MiraLAX takes to work include dosage, severity of constipation, cause of constipation, existing conditions, and individual digestion speeds.

  • To help MiraLAX work more effectively, follow the dosage instructions on the label, mix the powder with liquid, and stay hydrated.

  • MiraLAX is considered safe for most people but should not be used by those with certain medical conditions or allergies. Consult with your healthcare provider if constipation persists or worsens.

In this article:

If it’s been days since you’ve been able to go, you may visit your pharmacy looking for an over-the-counter (OTC) treatment to help move things along. One common OTC option is MiraLAX, an osmotic laxative used to treat occasional constipation. It works by drawing water into the colon, which softens the stool and makes it easier to pass, while also increasing the colon’s contents to trigger a bowel movement. 

Because MiraLAX does not directly stimulate the intestines like some laxatives, it can take longer to work. Here’s what you should know if you’re looking for relief.

RELATED: Foods to avoid when constipated

How long does it take for MiraLAX to work?

It usually takes 12 to 24 hours for MiraLAX (polyethylene glycol 3350) to take effect—according to Harish Gagneja, MD, a board-certified gastroenterologist at Austin Gastroenterology in central Texas—but it can take up to 96 hours in some cases.

Because it can take one to three days to work, it is normal not to feel immediate effects of MiraLAX. Unlike stimulant laxatives, MiraLAX might not even make you poop the same day you take it. 

“[The active ingredient, polyethylene glycol (PEG)] is not absorbed by the digestive system, so it remains in the intestine and draws water in the intestine,” Dr. Gagneja says. “This in turn increases the volume and water content of the stool, making it softer and more pliable. MiraLAX has no stimulant effect on the intestines.”

MiraLAX—which is also used off-label for bowel preparation before a colonoscopy—is formulated to offer gentle, gradual relief. It will probably take longer to work than other laxatives, but it also has fewer adverse effects. How quickly MiraLAX works can be impacted by the dosage, severity of constipation, and more. 

Factors that influence how long it takes for MiraLAX to work

There are several factors that impact how long it takes for MiraLAX to work, including:

  • Dosage: MiraLAX comes as a powder that you mix with liquid to consume. The recommended dose for occasional constipation is one pack (17 grams) dissolved in 4-8 ounces of liquid, which takes 24-96 hours to work, according to Jodie Pepin, Pharm.D., clinical pharmacy program director at Harbor Health in Austin, Texas. For patients prescribed to take MiraLAX for bowel preparation, Dr. Pepin recommends taking 238 grams (a medium-sized bottle) of MiraLAX with 64 ounces of Gatorade. “Taken along with a stimulant laxative called bisacodyl as directed prior to a colonoscopy, [MiraLAX] will generally start to work within one to three hours,” she says.
  • Severity of constipation: MiraLAX may take longer to work if you are severely constipated, says Dr. Gagneja. 
  • Cause of constipation: MiraLAX might take longer or be less effective if your constipation is caused by something other than dry, hard stool (such as medications or intestinal dysfunction).
  • Existing conditions: Chronic constipation or constipation from gastrointestinal conditions such as inflammatory bowel disease or intestinal obstructions can impact how long it takes for MiraLAX to work. 
  • Your average digestion speed: “‘Normal’ bowel movement frequency varies from individual to individual, as does … how long it takes food to pass through the colon, affecting the hardness or softness of the stool,” Dr. Pepin says.

Though MiraLAX can be administered at any time of day, some experts recommend taking it in the morning. This may increase your odds of having a same-day bowel movement. This type of laxative stops working once it is expelled from your digestive system, which should be within a few days.

RELATED: How long does constipation last?

How to use MiraLAX correctly

MiraLAX dosage depends on the form you are taking. Always follow the instructions on the label for correct dosing. 

  • For MiraLAX powder from a bottle, measure a single dose according to the measuring line on the bottle cap and mix with 8 ounces of liquid.
  • For MiraLAX packets, mix the entire single-dose packet with 4-8 ounces of liquid before drinking.

Once you have taken MiraLAX for constipation, Dr. Gagneja recommends staying hydrated and increasing your fiber intake to help the medication work. 

You can take a single dose of MiraLAX daily for up to seven days, but you should not take more medicine than indicated if you don’t think it’s working fast enough.

“MiraLAX is a safe medication, but taking it in excess quantities can cause diarrhea, electrolyte imbalance, and abdominal cramping,” Dr. Gagneja says. If you are still experiencing constipation after seven days of taking MiraLAX, stop the medication and consult a healthcare provider.

When not to use MiraLAX

MiraLAX is considered a safe medication, but there are a few people who shouldn’t use it. Both experts say to avoid taking MiraLAX if you:

  • Have had an allergic reaction to any component of MiraLAX or its generic polyethylene glycol 3350
  • Have or suspect a bowel obstruction 
  • Are experiencing severe abdominal pain
  • Have severe kidney problems or an electrolyte imbalance (a sign of kidney disease)

“There are no notable drug interactions with MiraLAX, but you should not take any medication within an hour prior to taking the MiraLAX,” Dr. Pepin adds.

MiraLAX side effects tend to be mild—gas, cramping, and nausea. If your constipation worsens or you develop new and severe gastrointestinal symptoms, stop using MiraLAX and consult your healthcare provider.

What if MiraLAX does not work for me? 

“It is okay to use or try other medications if MiraLAX is not working,” Dr. Gagneja says. Other OTC constipation treatments include Metamucil, a fiber supplement, and Dulcolax, a stimulant laxative. All of these treatments can cause common side effects like bloating and stomach cramps.

Ultimately, effective constipation treatment depends on the underlying cause of constipation. Constipation caused by irritable bowel syndrome may require a different treatment than constipation caused by diet or other prescription medications.

While MiraLAX is considered safe for occasional constipation, there are several factors to consider, including allergies, gastrointestinal conditions, and other medications or supplements you are taking. If you have any questions about using MiraLAX or another stool softener, consult a healthcare provider. You should also seek medical advice if constipation lasts more than a week or is accompanied by bloody stool, severe stomach pains, vomiting, or unexplained weight loss.

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