Milk of Magnesia and MiraLAX are not the same product, but they belong to the same broad category of laxatives. Both are osmotic laxatives, meaning they work by drawing water into the colon to soften stool and make it easier to pass. The key differences lie in their active ingredients, how quickly they work, their side effect profiles, and who can safely use them.
How Each One Works
Milk of Magnesia contains magnesium hydroxide, classified as a saline laxative. It pulls water into the intestines through an osmotic effect created by the magnesium salt. This typically produces a bowel movement within 30 minutes to 6 hours, making it a faster-acting option for occasional constipation.
MiraLAX contains polyethylene glycol 3350 (often shortened to PEG 3350), a synthetic compound that also holds water in the stool but works more gradually. It generally takes one to three days to produce a bowel movement. The effect is milder and more predictable, which is why many providers prefer it for ongoing constipation management.
Side Effects and Tolerability
MiraLAX tends to be gentler on the gut. It causes less gas and bloating than other osmotic laxatives, and most people tolerate it well at recommended doses. The main concern with prolonged use is the potential for electrolyte imbalances, where levels of minerals like sodium, potassium, and magnesium shift enough to cause symptoms like weakness, confusion, or irregular heartbeat.
Milk of Magnesia is more likely to cause cramping because it moves things along faster. It can also cause diarrhea if the dose is too high. Because the active ingredient is a magnesium salt, your body absorbs some of that magnesium. For most people with healthy kidneys, this isn’t a problem. But for anyone with reduced kidney function, magnesium can accumulate in the blood to dangerous levels, a condition called hypermagnesemia. Clinical guidelines for chronic kidney disease list magnesium-containing laxatives, including Milk of Magnesia, as products to avoid entirely.
Which One Is Better for Regular Use
Neither laxative is meant to be used indefinitely without guidance, but MiraLAX has a better safety profile for longer stretches. Overusing any laxative for weeks or months can reduce your colon’s natural ability to contract, making constipation worse over time. If you find yourself needing a laxative regularly, that’s a signal to talk with a provider about tapering off and restoring normal bowel function.
For short-term, occasional constipation where you want quick relief, Milk of Magnesia works faster. For a gentler, more gradual approach, especially if bloating or cramping is a concern, MiraLAX is typically the better choice.
Use in Children
Both products are used in pediatric constipation, but they’re ranked differently in clinical guidelines. MiraLAX is generally considered the first-line option for children, dosed by weight: about half a capful (8.5 grams) daily for children 10 to 20 kilograms, and one full capful (17 grams) daily for children over 20 kilograms, adjusted as needed.
Milk of Magnesia is typically a second-line choice for kids, partly because it’s more likely to cause cramping. Dosing is age-based: smaller amounts for children ages 2 to 5, increasing through adolescence. In both cases, the dose is adjusted until stool consistency reaches a comfortable range.
Forms and How You Take Them
Milk of Magnesia comes as a liquid, which has a chalky, slightly minty taste that some people find unpleasant. Chewable tablet versions also exist. You take it on its own, usually at bedtime or whenever constipation hits.
MiraLAX is a tasteless, odorless powder that dissolves in any beverage. This makes it easier to take consistently, especially for children or anyone who dislikes the taste of liquid laxatives. One capful is the standard adult dose, stirred into 4 to 8 ounces of water, juice, or another drink.
Quick Comparison
- Active ingredient: Milk of Magnesia uses magnesium hydroxide; MiraLAX uses polyethylene glycol 3350
- Speed: Milk of Magnesia works within hours; MiraLAX takes one to three days
- Stomach comfort: MiraLAX causes less bloating and cramping
- Kidney safety: MiraLAX is safe for people with kidney problems; Milk of Magnesia is not
- Form: Milk of Magnesia is a liquid or chewable tablet; MiraLAX is a dissolvable powder
- Pediatric preference: MiraLAX is first-line for children; Milk of Magnesia is second-line