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A study to evaluate the effects of rifaximin in patients with opioid use disorder

Fast replyRegistered drugNo placebo
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What is this trial about?

A plain-language summary of the goals, design and what participants do

This study investigates the use of rifaximin to treat opioid use disorder, a condition characterized by a problematic pattern of using opioid drugs. The research explores the microbiota-gut-brain axis, which is the complex communication system linking the bacteria in the digestive tract to the brain and behavior. The purpose of this study is to determine if treating the gut can help improve symptoms related to addiction.

Participants will receive either rifaximin or a placebo taken by mouth. During the trial, the focus is on how the treatment affects intestinal barrier integrity, which refers to how well the lining of the intestines prevents harmful substances from leaking into the body. The study also examines inflammatory signalling, which are the chemical messages sent by the body that can cause swelling or irritation in various tissues. Additionally, researchers will look at intestinal permeability, the measure of how easily substances pass through the intestinal wall, and fecal microbiota, which are the types and amounts of bacteria found in the stool.

The research process

The trial runs in 2 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    <b>medication administration</b>

    You will receive either the test medication or a placebo, which is a tablet that looks the same but contains no active medicine.

    If assigned to the test medication, you will take 800 mg of rifaximin (an antibiotic) by mouth.

    The medication administration will last for a duration of 4 weeks.

  2. Step 2

    <b>study monitoring</b>

    During the 4-week period, the study evaluates changes in intestinal barrier integrity, which refers to how well the lining of the intestines prevents harmful substances from passing through.

    The study also monitors gut-derived inflammatory signalling, which are the chemical messages sent by the body in response to inflammation in the digestive tract.

    Measures are taken regarding the intensity of opioid craving, social isolation, and the tapering of methadone therapy (the gradual reduction of methadone dosage).

Who can join the trial?

7 criteria

  • You must have a diagnosis of opioid use disorder, which is a medical condition involving the problematic use of opioids (such as pain medications or other substances).
  • Your diagnosis must meet the specific standards set by the DSM-5, which is a manual used by doctors to identify and classify mental health and substance use conditions.
  • You must be between 18 and 60 years old.
  • If you are female, you must not be pregnant and not be breastfeeding.
  • You must be able to understand the study details and provide informed consent, which means signing a document that shows you agree to participate after being fully told about the study.
  • The study prefers people who have recently stopped using opioids (for example, within the last 1 to 4 weeks) to study how the body adapts, but you cannot be in a state of uncontrolled acute withdrawal, which refers to the severe and unpredictable physical symptoms that happen when a person suddenly stops using a substance.
  • If you are currently on methadone maintenance treatment (a medication used to reduce symptoms of opioid use), you can participate as long as you have been taking a stable dose and agree not to change that dose during the first month of the study.

Who cannot join the trial?

10 criteria

  • Having known organic gastrointestinal diseases, which means physical diseases affecting the digestive tract, such as Crohn’s disease or ulcerative colitis (conditions that cause swelling and irritation in the gut), untreated celiac disease (an immune reaction to eating gluten), advanced-stage liver cirrhosis (permanent scarring of the liver), or gastrointestinal malignancies (cancer of the digestive system) that could change how well the intestines work or make a biopsy (a procedure where a tiny piece of tissue is removed for testing) unsafe.
  • Having a diagnosis of an active chronic infection, which is a long-lasting infection caused by germs.
  • Having severe uncontrolled medical conditions, which are serious health problems that are not being managed effectively by treatment.
  • Having active psychiatric disorders, which are mental health conditions that might make it difficult to follow the study rules.
  • Currently using antibiotics (medicine used to kill bacteria) or probiotics (helpful bacteria) that change the microbiota (the community of tiny organisms living in your gut), unless these have been stopped for at least 4 weeks.
  • Having HIV/AIDS or being in an immunocompromised status, which means having a weakened immune system that cannot fight infections well.
  • Having a known allergy or a severe bad reaction to rifaximin or other medicines in the rifamycin class of antibiotics.
  • For women, being pregnant or planning to become pregnant during the study.
  • Taking part in another interventional clinical trial, which is a different medical study involving a treatment or test.
  • Being unable to follow the specific rules and steps required by the study.
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Investigated drugs

Rifaximin is an antibiotic taken by mouth that works directly in the gut to target bacteria. In this study, it is being tested to see if it can help improve the health of the intestinal lining and reduce inflammation in the digestive system of people with opioid use disorder.

What is already known about the treatment

  • Rifaximin

    This medication is an antibiotic taken orally in the form of a film-coated tablet. It is currently used in medicine to treat various digestive issues and is being studied for its potential effects on the gut-brain connection. It works by stopping the growth of harmful bacteria in the intestines without being widely absorbed into the rest of the body. This drug is classified as a gastrointestinal antibiotic.

  • Placebo

    This is an inactive tablet taken orally that looks and feels exactly like the real medication to ensure the study remains fair and unbiased. It contains no active medicine and is only made of common ingredients used to hold pills together. Because it has no medicinal effect, it is used as a control to compare against the actual treatment.

Investigated diseases

Substance use disorder - This condition involves a pattern of using a substance, such as opioids, in a way that leads to significant problems or distress. It is characterized by a strong urge or craving to consume the substance. Over time, an individual may find it difficult to control their usage despite wanting to stop. The disorder can also lead to social disconnection and isolation from others. It often progresses as the dependency on the substance becomes more ingrained in a person's daily life.
Trial detailsLast updated 2 Oct 2026
Age18+ yearsPhasePhase IITrial ID2025-524778-41-00Estimated enrolment50 patientsSponsorFondazione Santa Lucia

sourced from the EU Clinical Trials Register and site verification

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