Lactulose and MiraLax (polyethylene glycol 3350) are roughly equal in strength for treating constipation. Both are osmotic laxatives that work the same way, pulling water into the colon to soften stool, and both typically produce a bowel movement within one to three days. The real differences between them come down to side effects, taste, cost, and convenience rather than raw potency.
How Both Laxatives Work
Lactulose and MiraLax belong to the same category of laxative. Osmotic laxatives are compounds your body can’t absorb, so they stay in the intestine and draw water in behind them. As water collects in the colon, stool becomes softer and easier to pass. Neither one stimulates the muscles of the intestine to contract, which is why they’re gentler than stimulant laxatives but also slower to kick in.
The chemical difference is straightforward. Lactulose is a synthetic sugar that gut bacteria partially break down, producing acids that further help move things along. MiraLax is a large, inert polymer that passes through the digestive tract without being broken down at all. That bacterial fermentation step is what gives lactulose its most notable drawback: gas.
Side Effects Favor MiraLax
Because gut bacteria ferment lactulose, it causes significantly more flatulence. In patient-reported data compiled by Drugs.com, about 26% of lactulose users reported gas compared to roughly 9% of MiraLax users. Bloating and cramping were similar between the two, each affecting around 11% of users. For many people, that threefold difference in gas is the deciding factor, especially when both laxatives relieve constipation at comparable rates.
MiraLax can cause abdominal distension, nausea, and loose stools, but these tend to be milder overall. Neither laxative carries serious risks for most adults when used at standard doses for short periods.
Taste and Ease of Use
Lactulose is a thick, sweet syrup. Some people find the sweetness tolerable, but others describe it as cloying, particularly when taking it twice a day. The standard adult dose is 15 milliliters twice daily.
MiraLax is a flavorless, odorless powder that dissolves completely in water, juice, coffee, or any beverage. This flexibility is a major practical advantage. The standard adult dose is 17 grams (one capful) mixed into about 8 ounces of liquid, taken once a day. The fact that it virtually disappears into a drink makes long-term adherence easier for most people, and it’s a big reason MiraLax became so popular after switching to over-the-counter status in 2006.
Prescription vs. Over the Counter
MiraLax is available without a prescription in the United States. You can pick it up at any pharmacy or grocery store, and generic versions of polyethylene glycol 3350 are widely available at lower cost. Lactulose still requires a prescription in the U.S., which adds a step and can mean higher out-of-pocket costs depending on your insurance. In some other countries, lactulose is sold over the counter, but in the American healthcare system this difference in access matters.
Where Lactulose Has a Unique Role
Lactulose has one important use that MiraLax doesn’t share: treating hepatic encephalopathy, a condition where the liver can’t clear ammonia from the blood and it builds up to levels that affect brain function. Lactulose reduces ammonia production and absorption in the gut, which is why it’s prescribed at much higher doses (30 to 50 milliliters, three times daily) for patients with liver disease.
Interestingly, research published in BMJ Open Gastroenterology found that a single dose of polyethylene glycol actually improved mental status scores faster and shortened resolution time compared to lactulose in hospitalized patients with hepatic encephalopathy. The pooled data showed patients given polyethylene glycol resolved about 1.5 days sooner. However, lactulose produced a greater drop in blood ammonia levels at the 24-hour mark, suggesting the two drugs may work through somewhat different pathways. Lactulose remains the standard treatment for this condition, but the research hints that polyethylene glycol could play a role in acute settings.
Which One to Choose for Constipation
For everyday constipation, MiraLax is the more practical choice for most people. It causes less gas, dissolves invisibly into drinks, doesn’t require a prescription, and works on the same timeline. Lactulose isn’t “stronger” in any meaningful clinical sense. Both produce similar improvements in stool frequency and consistency within one to three days.
Lactulose may still be the right option if your doctor prescribes it for a specific reason, such as liver-related ammonia issues, or if you’ve tried MiraLax without success and want to see whether the added fermentation effect makes a difference for your gut. Some pediatric guidelines include both as first-line options for children, with dosing adjusted by age. For kids, lactulose can be started as low as 2.5 milliliters twice daily in infants, while MiraLax dosing is typically weight-based and determined by a pediatrician.
If you’ve been using either laxative regularly for more than two weeks without improvement, that’s worth bringing up with a healthcare provider. Chronic constipation sometimes points to underlying causes that an osmotic laxative alone won’t fix.