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What is lactulose?

Lactulose is a derivative of lactose. It is based on synthetic sugar.

Your doctor may prescribe lactulose to treat constipation. It breaks down in the colon into products that help retain water in the stools. This softens them and eases constipation symptoms.

Lactulose can also reduce blood ammonia levels in people with liver disease by drawing ammonia from the blood and bringing it into the colon. From here, the body removes it safely via the stools.¹

What is lactulose used to treat?

Lactulose is used to treat chronic constipation. It also aids in the prevention and treatment of hepatic encephalopathy.

You don’t absorb lactulose when it enters your body. Instead, it travels to the colon without changing its form. Bacteria in the colon break lactulose down into compounds that acidify the fecal contents. Acidification leads to increased osmotic pressure in the colon, which increases the stools’ water content and softens them. This makes them easier to pass.

Lactulose also eliminates ammonia from the body, which helps prevent hepatic encephalopathy. Hepatic encephalopathy results from dangerous blood ammonia levels caused by liver disease.

The drug keeps ammonia from entering the bloodstream by making the contents of the intestine more acidic than the blood. Ammonia remains in the colon and leaves the body along with feces and other waste.

Dosage forms and strengths

Lactulose usually comes as a syrup or solution to be taken orally or given rectally. It is also available in powder form, which can be diluted according to the packet instructions to make a solution.

The strength of the formula is typically as follows:

  • Syrup (solution): 10g/15mL

  • Powder: 10g or 20g per packet

How do you take lactulose?

You can take lactulose rectally or orally.

Here are the recommended adult doses for oral use:²

  • For constipation: a dose of 15–30mL (or 10–20g of powder) taken daily, increasing to a maximum of 60mL (or 40g of powder) per day until stools are soft and regular

  • For patients with acute hepatic encephalopathy: 30–45mL (20–30g of powder), repeated every 1–2 hours until you pass 2–3 soft stools. After that, the dose can be adjusted to achieve a bowel movement 2–3 times per day.

The drug can be administered rectally by enema if there is a risk of aspiration when taking medication orally. This would be given by a doctor as a treatment for hepatic encephalopathy. You would not be able to self-administer the medication.

Seeing results

Lactose can provide results in a couple of hours. For many people, it takes up to 48 hours for the medication to work.³

When recommending a course of treatment, your doctor will take into account that the medication may not work immediately. Therapy for both constipation and hepatic encephalopathy may take several days to years.

If you don’t notice any results after 48 hours or the time specified by your doctor, speak to them about adjusting your treatment.

Who should not take lactulose?

Lactulose may not be safe for everyone. It is contraindicated in the following groups:⁴

  • Galactosemia: Patients with galactosemia should avoid taking this laxative as it contains galactose. Tell your doctor if you need to follow a galactose-free diet.

  • Diabetes: Even though only a small amount of lactulose undergoes systemic absorption, patients with diabetes should exercise caution when taking this medication. Lactulose was thought to be able to raise blood sugar levels, but recent research points to a role for lactulose as a prebiotic in the prevention and treatment of type 2 diabetes.⁵

  • Older adults: Lactulose can cause nausea. Older adults may be given an alternative laxative.

  • Pregnancy: Prolonged use of lactulose during pregnancy could cause an electrolyte imbalance.⁶

Avoid lactulose if you are planning to have surgery or any procedure using cauterization. The drug can generate flammable gas (hydrogen gas) that can explode when it comes in contact with a spark.⁷

Potential side effects of lactulose

Lactulose could cause any of the following side effects:⁸

  • Loose stools

  • Excessive bowel activity

  • Nausea

  • Vomiting

  • Abdominal cramping and distension

  • Belching

  • Flatulence (gas)

The drug can also cause severe side effects, particularly if your dose is too high. These include any of the following:

  • Dehydration

  • Severe diarrhea

  • Electrolyte disorders

  • Metabolic acidosis

  • Hypernatremia (high sodium levels in the blood)

  • Hypokalemia (low potassium levels in the blood)

It is quite common to experience some cramping and gas when you first start taking lactulose, but these effects usually improve. However, side effects such as dehydration, vomiting, and stomach pain or severe cramping and diarrhea require medical attention. Stop using the medication and contact your doctor immediately.


Taking an excessive amount of lactulose could cause such symptoms such as the following:

  • Severe abdominal pain

  • Diarrhea

  • Nausea and vomiting

  • Dehydration

  • Electrolyte and metabolic disturbances

If you overdose on lactulose, contact your doctor immediately or call 911. You can also reach out to the National Poison Control Help Line on 1-800-222-1222.⁹

Allergy information

Allergic reactions to lactulose are rare.

Symptoms of an allergic reaction to lactose include the following:

  • Hives

  • Breathing problems

  • Swelling of the face, throat, lips, or tongue

  • Vomiting

  • Loss of consciousness

If these symptoms occur, call your doctor or 911 to get emergency medical assistance.

Long-term use of lactulose

Doctors usually prescribe lactulose for shorter periods to treat severe constipation. You will likely take the medication until you have a bowel movement. This rarely takes longer than a week. In some cases, the dose will be continued for a longer time to help you pass stools regularly.

Taking lactulose for a long time increases your risk of experiencing side effects.¹⁰

If you are an older patient taking lactulose for over three months, you may develop electrolyte imbalances. Your doctor will likely monitor you closely with regular lab tests or recommend a different medication.

For the prevention or treatment of hepatic encephalopathy, your doctor may recommend taking lactulose for many months.¹¹

In all cases, it’s important to talk to your doctor about the expected duration of treatment.

Pregnancy category

The US Food and Drug Administration (FDA) has categorized lactulose as a pregnancy category B medication.¹² This means there’s no evidence from animal studies that the drug harms a fetus.

Some sources state there is likely little risk for a fetus as the drug mostly remains in the colon and absorbs very minimally into the body.¹³

Lactulose and pregnancy

Continuous use of lactulose could cause diarrhea and dehydration, which may affect the fluids and nutrition the mother and fetus receive.

Doctors prescribe lactulose during pregnancy only if its benefits outweigh the possible risks. It’s vital to seek medical advice about taking lactulose during pregnancy and if you’re planning to become pregnant.

The labeling on lactulose products advises taking the medication with caution while breastfeeding. However, the potential risk to the infant is very low. Minimal amounts are absorbed into the mother’s bloodstream, so it is unlikely to be present in breast milk. Occasional administration for constipation treatment isn’t likely to cause harm.¹⁴

Missed doses

Keep your lactulose intake evenly spaced throughout the day. If you miss a dose, take it as soon as you remember. However, if there are only 1–2 hours until the time you need to take the next one, skip it and continue taking medication according to your usual schedule. Don’t take a double dose.

Drug interactions

The following drugs can interact with lactulose:¹⁵

  • Antacids like aluminum hydroxide (AlternaGEL), omeprazole (Prilosec), and calcium carbonate (Tums, Maalox) — change the pH, making lactulose less effective

  • Antibiotics such as cephalexin (Keflex), azithromycin (Zithromax), and ciprofloxacin (Cipro) — take with caution as their action on colonic bacteria may make lactulose less effective

  • Other laxatives such as bisacodyl (Dulcolax), docusate (Colace), magnesium citrate, psyllium (Metamucil), polyethylene glycol (Miralax), mineral oil, and castor oil — may cause more loose stools and might interrupt hepatic encephalopathy treatment

  • Loop diuretics, like bumetanide (Bumex), furosemide (Lasix), and ethacrynic acid (Edecrin)

  • Supplements, such as calcium phosphate

  • Dichlorphenamide (Keveyis) — can increase metabolic acidosis with severe diarrhea

  • Opiate medications such as hydrocodone — may be less effective due to decreased absorption of the drug

  • Lithium — case reports of lithium toxicity

Can I drink alcohol while taking lactulose?

There is no contraindication for drinking alcohol when taking this medication, unless you are taking it for liver disease. However, too much alcohol mixed with lactulose could cause dehydration and diarrhea.

What to discuss with your doctor before starting lactulose

  • Tell your doctor if you have any allergies to lactulose.

  • Discuss prescription and non-prescription medications, herbs, and supplements you are using (whether daily or occasionally).

  • Tell your doctor if you have diabetes, inflammatory bowel disease, or follow a low-galactose diet.

  • Let your doctor know if you are pregnant or breastfeeding. Discuss any plans for pregnancy.

  • Share plans for any upcoming surgeries or medical procedures.

Stopping lactulose

Lactulose doesn’t cause addiction or tolerance. Your doctor should tell you when it’s time to stop using the medication.

Drug approval history

The US FDA approved lactulose in 1977.¹⁶

Tips for taking lactulose

The following tips can help you take the drug safely and see the best results:

  • If your doctor prescribes lactulose twice a day, make sure you take it in evenly spaced doses.

  • Tell your doctor if you don’t see an improvement in your symptoms after 48 hours. They may adjust the dosage or replace the medication.

  • You can mix your dose with half a glass of water, milk, or fruit juice to improve the taste.

  • Drinking lots of water can help ease constipation alongside lactulose.

  • If you are taking lactulose for constipation, speak to your doctor about adding high-fiber foods to your diet, including wholegrain bread and cereals, fruits, vegetables, and brown rice.

  • Exercise daily to help increase bowel activity.

  • Store your medicine at room temperature.

Lactulose FAQs

What is lactulose used for?

Lactulose is used to treat constipation and hepatic encephalopathy.

What are the side effects of lactulose?

Common side effects of lactulose are diarrhea, nausea, gas and bloating, and stomach pain.

When should you not use lactulose?

Before taking lactulose, tell your doctor if you have diabetes, are taking antacids or other laxatives, are following a low-galactose diet, or might be pregnant.

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.