What Is Lactulose? Uses, Dosage, and What You Need to Know in 2026

Lactulose is a prescription medication used to treat two very different conditions: chronic constipation and hepatic encephalopathy (a brain condition caused by liver disease). It's been in clinical use since 1966, appears on the World Health Organization's List of Essential Medicines, and is one of the most prescribed laxative medications in the United States — with over 900,000 prescriptions filled in 2023 alone.

What Is Lactulose?

Lactulose is a synthetic (man-made) sugar — a disaccharide made from galactose and fructose — that your small intestine cannot digest or absorb. Because it passes through your upper GI tract unchanged, it arrives in the colon intact, where it plays its therapeutic role.

It's classified as an osmotic laxative and colonic acidifier. Despite containing sugar-based compounds, lactulose is not nutritionally absorbed and does not contribute meaningful calories to your diet.

What Is Lactulose Used For?

1. Chronic constipation: Lactulose powder (Kristalose) is FDA-approved for this indication. It increases the number of days on which bowel movements occur and increases stool frequency. It is often used when lifestyle changes (diet, hydration, exercise) and first-line OTC options haven't worked.

2. Hepatic encephalopathy (HE): Lactulose solution is FDA-approved for preventing and treating portal-systemic encephalopathy, including hepatic pre-coma and coma. When the liver can't filter toxins properly, ammonia builds up in the blood and affects brain function — causing confusion, mood changes, and in severe cases, unconsciousness. Lactulose reduces ammonia production and absorption in the gut.

Off-label uses: Lactulose is also sometimes used for opioid-induced constipation, pediatric functional constipation, and as part of the SIBO (small intestinal bacterial overgrowth) breath test — though the reliability of the latter has been questioned.

Lactulose Brand Names and Formulations

Lactulose is available in the following forms:

• Generic lactulose oral solution (10 g/15 mL): Also sold as Enulose, Constulose, Generlac. Available in 237 mL, 473 mL, and 946 mL bottles. Colorless to amber syrup; can be mixed with juice, milk, or water.
• Kristalose (lactulose crystalline powder): Available in 10 g and 20 g single-dose packets. Made by Cumberland Pharmaceuticals. No generic available. Dissolves in water.
• Rectal solution: Used in hospital settings for acute hepatic encephalopathy; not routinely available at retail pharmacies.

Lactulose Dosage Guide

For Constipation (Adults):

• Usual starting dose: 15–30 mL (10–20 g) orally once daily
• Maximum dose: up to 60 mL/day
• Onset of action: 24–48 hours — don't be surprised if it takes a day or two to work

For Hepatic Encephalopathy (Adults):

• Starting dose: 30–45 mL (20–30 g) 3–4 times daily
• Goal: 2–3 soft bowel movements per day (not diarrhea)
• Acute HE: may be dosed every 1–2 hours initially until catharsis is achieved

For Children (Constipation):

• Infants: 2.5–10 mL/day in 3–4 divided doses
• Children/adolescents: 40–90 mL/day in divided doses, adjusted to produce 2–3 soft stools daily

How to Take Lactulose

• Mix the liquid with water, milk, or fruit juice to improve the taste — it's quite sweet on its own
• For Kristalose powder: mix the entire packet content in at least 4 oz (half a glass) of water until dissolved
• Use a proper measuring device (not a kitchen spoon) for liquid doses — accuracy matters, especially for HE treatment
• Store at room temperature — the liquid may darken over time, which is normal and doesn't affect potency
• Do not freeze

Who Should Not Take Lactulose?

Lactulose is contraindicated in people with galactosemia — a condition requiring a low-galactose diet — because lactulose solution contains small amounts of galactose. People with diabetes should use lactulose with caution and inform their provider, as the galactose and lactose content may affect blood glucose.

For a full overview of side effects and warnings, see our guide on lactulose side effects.