Does Lactulose Interact With Medication?

In our latest question and answer, our pharmacist discusses potential drug interactions with the laxative lactulose.

Dec 21, 2017

Max asked

Can I use lactoluse solution for a SIBO Test while on Lexapro and Synthroid? Any drug interactions?


Lactulose, a derivative of lactose (but chemically distinct), is a commonly used prescription medication for the treatment of constipation and hepatic encephalopathy. When taken orally, lactulose is not digested by any gastrointestinal enzyme. Therefore, doses of the drug go through the intestines and reach the colon virtually unchanged.

While we do not digest or absorb lactulose, the bacteria in our gut does. The normal bacterial flora in our gut degrade lactulose into lactic acid as well as formic and acetic acid. In addition, the bacteria that feed on lactulose will produce excess hydrogen and methane gas. These gasses are detectable in your breath and this is what the SIBO test is designed to look for. Your breath will be sampled for levels of hydrogen and methane, giving an indication of possible bacterial overgrowth.

As a laxative, lactulose increases pressure in the GI tract, causing fluid accumulation that softens the stool and distends the colon, which stimulates contractions. This accelerates evacuation of the stool.

In terms of drug interactions, lactulose doesn't interact with most medications. There may be a slight decrease in the absorption of some drugs, but it typically isn't significant. Unlike most laxatives, the effects of lactulose only occur in the colon, not in the small or large intestine.  Due to this, the laxative effects can take one to two days to occur (the time it takes lactulose to reach the colon).

With levothyroxine specifically, it is generally recommended to take it first thing in the morning, with no other food or drugs. Any change in absorption of levothyroxine can have significant effects. Taking it with lactulose most likely won't have a large effect, but it could have some effect nonetheless and should be separated.

Lastly, below are some of the common drug interactions with lactulose that should be avoided:


It is thought that a sufficiently low pH (i.e. high acid) is needed for lactulose to have effect. Antacids (e.g. Tums) raise pH and therefore could make lactulose less effetive.


Drugs used to treat constipation (e.g. lactulose) can counteract the effect of antidiarrheals and therefore should not be administered concurrently.


Other laxatives, such as Senna and bisacodyl should not concurrently with lactulose.

Loop Diuretics

Loop diuretics, such as furosemide and torsemide can increase the risk of hypokalemia (low blood potassium). Potassium levels should be monitored if necessary.

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