Lactulose and Miralax are not the same medication. They belong to the same category of laxatives and work in a similar way, but they differ in their chemical makeup, how you get them, and what they’re used for. Understanding these differences can help you figure out which one fits your situation.
How They Compare at a Glance
Both lactulose and Miralax are osmotic laxatives, meaning they pull water into the intestines to soften stool and stimulate bowel movements. That shared mechanism is why people often assume they’re interchangeable. But the similarities mostly end there.
Lactulose is a synthetic sugar (a disaccharide) that your body can’t digest or absorb. It passes through to the colon, where bacteria break it down and create byproducts that draw water into the bowel. Miralax’s active ingredient is polyethylene glycol 3350 (PEG 3350), a large polymer molecule that also draws water into the intestines but does so without being broken down by gut bacteria. This distinction matters because the bacterial fermentation of lactulose produces gas, which is a key reason the two medications feel different to take.
Prescription vs. Over the Counter
One of the biggest practical differences is how you get each one. Miralax is available over the counter in the United States as a tasteless, odorless powder you mix into any beverage. Lactulose, on the other hand, requires a prescription. It comes as a sweet syrup, a liquid solution, or a powder for reconstitution. A typical adult dose of lactulose is 15 to 30 mL (one to two tablespoons) per day, though a doctor may adjust this up to 60 mL daily depending on the condition being treated.
How Fast They Work
Neither medication provides instant relief. Miralax generally produces a bowel movement within one to three days of regular use. Lactulose works on a similar timeline, typically taking a couple of days before you notice results. The NHS advises contacting a doctor if lactulose hasn’t worked after three days. Both are designed for daily use rather than as quick-acting rescue laxatives.
Side Effects and Tolerability
Because lactulose is fermented by gut bacteria, it tends to cause more bloating, gas, and cramping than Miralax. Nausea and abdominal discomfort are also possible with lactulose. Miralax bypasses bacterial fermentation entirely, so it generally causes less gas and is considered easier on the stomach. In clinical trials comparing the two, side effects for both were mild (occasional diarrhea, abdominal pain, or vomiting), but patients using PEG-based products consistently report fewer complaints related to bloating.
This difference in tolerability is one reason many doctors and patients prefer Miralax for routine constipation, especially when long-term use is expected.
Effectiveness for Constipation
Head-to-head studies show that both medications are effective and safe treatments for chronic constipation, with no significant difference in overall outcomes. However, some research suggests PEG may work slightly faster. A study of 113 pregnant women with constipation found that both polyethylene glycol and lactulose improved symptoms equally by the end of a three-week course, but polyethylene glycol showed a faster therapeutic effect during the first two weeks of treatment.
Lactulose Has a Use Miralax Doesn’t
This is the most important clinical distinction between the two. Lactulose has a well-established role in treating hepatic encephalopathy, a condition where the liver can’t adequately clear ammonia from the blood, leading to confusion, disorientation, and in severe cases, coma. Miralax has no role in this condition.
Lactulose reduces blood ammonia levels by 25% to 50% through several mechanisms unique to its chemistry. When bacteria ferment lactulose in the colon, they lower the local pH. This acidic environment converts ammonia into ammonium, an ionized form that can’t cross back into the bloodstream. The lower pH also kills ammonia-producing bacteria in the gut and blocks certain enzymatic pathways that generate ammonia from dietary amino acids. On top of all this, colonic bacteria use the trapped ammonia as a nitrogen source for their own growth, effectively removing it from circulation.
This is why lactulose remains a prescription medication. For someone with liver disease, it’s not just a laxative. It’s a core treatment that prevents a potentially life-threatening buildup of toxins in the brain.
Which One Is Right for You
If you’re dealing with occasional or chronic constipation and want something you can pick up at the pharmacy without a prescription, Miralax is the more accessible and generally better-tolerated option. It mixes easily into drinks, causes less bloating, and works within a similar timeframe.
If your doctor has prescribed lactulose specifically, there’s likely a reason. You may have a liver condition that benefits from lactulose’s ammonia-lowering properties, or your doctor may have other clinical reasons for choosing it. Switching to Miralax on your own wouldn’t provide the same benefits in that scenario.
Both medications are considered safe during pregnancy, and both are used in children under medical supervision. For most people with straightforward constipation, the two produce equivalent results, so the choice often comes down to side effects, cost, and convenience.