Lactulose or PEG for Constipation?The Four-Week Trial

What the head-to-head study found

In a 1999 randomized trial, 115 people with chronic constipation entered a comparison of PEG 3350 with electrolytes and lactulose. Ninety-nine completed four weeks: 50 in the PEG group and 49 in the lactulose group.¹

The published abstract reports more stools, less straining and greater overall improvement with PEG. Gas was reported less often with PEG, although overall clinical tolerance was similar. PEG produced more liquid stools, with a statistically significant difference only during the first two weeks.¹

Doses could change according to response. The study tested particular sachet formulations; its findings do not make every constipation product interchangeable or establish a personal dosing schedule.¹

How the later guideline distinguishes them

The 2023 AGA/ACG guideline for adults with chronic idiopathic constipation strongly recommends PEG, with moderate certainty of evidence. Its lactulose recommendation is conditional, with very low certainty, for people whose over-the-counter treatments have failed or caused intolerance.²

That difference describes the evidence behind the recommendations. It does not mean that a person doing well on prescribed lactulose must switch.

The guideline identifies dose-dependent bloating and gas as common limitations of lactulose. PEG can also cause abdominal swelling, loose stools, gas or nausea. If bloating is already your main complaint, include it in the treatment goal rather than judging success only by bowel-movement counts.²

Give the review a stool record and a symptom record

Write down the product name, amount taken and time. For each bowel movement, note stool form, straining and whether evacuation felt complete. Record gas or bloating separately, including when it began relative to treatment.

Bloated by Dinner includes a Belly Clock log with bowel-movement times, Bristol stool types, medicines and digestive symptoms. Add the laxative details to that record. Bring both the log and the packaging to a pharmacist or clinician.

Useful questions include:

• “Is the stool still hard, or am I straining despite softer stool?”
• “Would you adjust this treatment or choose a different one?”
• “What should I do if I develop diarrhea, and when should we review the response?”

Lactulose takes days to start working

The NHS³ page, reviewed in September 2026, says lactulose can take two to three days to start working. It advises seeing a doctor if symptoms have not improved or have worsened after more than three days of use. An unchanged first evening is not a reason to keep adding doses.³

Use the supplied measuring cup or spoon. The NHS³ advises discussing bothersome or persistent side effects with a pharmacist or doctor, who may suggest reducing the dose. It also warns that frequent diarrhea from excessive, prolonged use can lower potassium. Keep the dose and symptoms visible in the review.³