Miralax is a laxative, not a stool softener. Specifically, it’s classified as an osmotic laxative, which works differently from both traditional stool softeners like Colace and stimulant laxatives like Dulcolax. The confusion is understandable because Miralax does soften stool as part of how it works, but its mechanism and strength place it firmly in the laxative category.
How Miralax Works
Miralax’s active ingredient is polyethylene glycol 3350 (PEG 3350), a polymer-based compound that draws water into the colon. That extra water does two things: it softens stool, and it increases pressure inside the intestines, which triggers the intestinal muscles to push stool along. This dual action is what distinguishes an osmotic laxative from a simple stool softener.
It typically takes one to three days to produce a bowel movement. You dissolve 17 grams of powder (one capful) in 4 to 8 ounces of any beverage, hot or cold, and drink it once daily. The over-the-counter label says not to use it for more than seven days without medical guidance.
How Stool Softeners Differ
Stool softeners like docusate sodium (the active ingredient in Colace) work by increasing the amount of water absorbed directly into the stool itself. Think of them as a gentle moisturizer for hard, dry stool. They don’t stimulate the intestinal muscles to contract, so they’re milder and slower-acting than osmotic laxatives.
Because stool softeners don’t push things along, they’re often recommended in situations where straining is the main concern rather than infrequent bowel movements. People recovering from surgery, dealing with hemorrhoids, or managing anal fissures tend to benefit most from stool softeners. If your main problem is that you simply aren’t having bowel movements often enough, an osmotic laxative like Miralax is generally more effective.
Types of Laxatives at a Glance
- Osmotic laxatives (Miralax): Draw water into the colon, softening stool and stimulating bowel contractions. Work in one to three days.
- Stool softeners (Colace): Add moisture to stool so it’s easier to pass. Don’t actively stimulate the bowel. Best for preventing straining.
- Stimulant laxatives (Dulcolax, Senokot): Directly trigger the muscles in the intestinal wall to contract. Faster-acting but more likely to cause cramping.
- Bulk-forming laxatives (Metamucil, Benefiber): Add fiber that absorbs water, increasing stool size so the bowel pushes it through naturally.
Common Side Effects
Most people tolerate Miralax well. The most frequently reported side effects are bloating, gas, stomach fullness, and mild nausea. These tend to show up early and often improve after a day or two of use.
Because Miralax pulls water into the colon, there’s a small risk of dehydration or electrolyte imbalance, particularly in older adults or anyone with kidney or heart problems. Signs to watch for include decreased urination, dizziness, headache, unusual thirst, or vomiting. A fast or irregular heartbeat, muscle cramps, or tingling in the hands and feet can signal an electrolyte issue that needs medical attention.
Using Miralax Longer Than Seven Days
The OTC label caps use at seven consecutive days, but many healthcare providers prescribe Miralax for longer stretches in people with chronic constipation. If you find yourself reaching for it regularly beyond a week, that’s a conversation worth having with your provider. They can rule out underlying causes and determine whether extended use is appropriate for your situation. PEG 3350 doesn’t cause the kind of bowel dependency associated with stimulant laxatives, which is one reason providers are often comfortable with longer courses.
Miralax for Children
The OTC label says to ask a doctor before giving Miralax to anyone 16 or younger. That said, pediatric gastroenterologists frequently recommend it for childhood constipation, and major guidelines from organizations like the North American Society for Pediatric Gastroenterology support its use at weight-based doses. The typical starting range for children is 0.4 to 0.8 grams per kilogram of body weight per day, adjusted up or down based on how the child responds. For a child dealing with a more severe backup (fecal impaction), providers may use higher doses for three to six days before stepping down to a maintenance amount.
Pediatric dosing varies enough from child to child that it genuinely needs to be guided by a provider rather than guessed at from the adult instructions on the bottle.
Choosing Between Miralax and a Stool Softener
If you’re dealing with occasional constipation and your bowel movements are simply infrequent or require significant effort, Miralax is the stronger option. It both softens stool and gets the bowel moving. If your stools are hard and painful to pass but you’re still going regularly, a stool softener may be enough and comes with fewer side effects. Some people use both together on medical advice, since they work through different mechanisms and don’t interfere with each other.