Xifaxan 550 mg is an antibiotic tablet used for two FDA-approved conditions: reducing the risk of hepatic encephalopathy (HE) recurrence in people with liver disease, and treating irritable bowel syndrome with diarrhea (IBS-D). It’s also frequently prescribed off-label for small intestinal bacterial overgrowth (SIBO). What makes Xifaxan unusual among antibiotics is that less than 0.4% of the drug gets absorbed into the bloodstream. It stays almost entirely in the gut, targeting bacteria where the problem is while leaving the rest of the body largely untouched.
How Xifaxan Works in the Gut
Most antibiotics are designed to enter the bloodstream and travel throughout the body. Xifaxan does the opposite. After you swallow a tablet, 80% to 90% of it concentrates in the intestines, with less than 0.2% reaching the liver and kidneys and virtually nothing reaching other tissues. The drug passes through the digestive tract and is excreted almost entirely unchanged in stool.
This gut-focused design is the reason Xifaxan works well for intestinal conditions but would be useless for something like a lung or urinary tract infection. It reduces harmful bacterial activity in the intestines without the widespread effects that systemic antibiotics often cause.
Hepatic Encephalopathy Prevention
Hepatic encephalopathy is a serious complication of advanced liver disease. When the liver can’t filter toxins properly, ammonia and other waste products build up in the blood and affect brain function. This can cause confusion, disorientation, personality changes, and in severe cases, coma. Episodes tend to recur, and each one can mean a hospital stay.
For HE prevention, Xifaxan 550 mg is taken twice daily on an ongoing basis, typically alongside lactulose (a laxative that also helps remove ammonia). The goal isn’t to treat an active episode but to keep them from coming back. In clinical trials, the combination of Xifaxan plus lactulose reduced the risk of breakthrough HE episodes by up to 66% compared to lactulose alone. That benefit held across different levels of liver damage: patients with mild, moderate, and severe cirrhosis all saw significant reductions in recurrence. Among patients taking Xifaxan, roughly 17% to 29% experienced a breakthrough episode depending on how advanced their liver disease was, compared to 44% to 64% on placebo.
Irritable Bowel Syndrome With Diarrhea
For IBS-D, the approach is completely different from the HE use. Instead of continuous daily treatment, Xifaxan is taken as a short course: one 550 mg tablet three times a day for 14 days. The idea is to reset the bacterial environment in the gut, which can reduce bloating, abdominal pain, and loose stools.
If symptoms return after a course of treatment, you can repeat the same 14-day regimen up to two more times, for a maximum of three total courses. There’s no fixed schedule for retreatment. It’s based on whether and when symptoms come back. Some people get lasting relief from a single course, while others need additional rounds.
Off-Label Use for SIBO
Small intestinal bacterial overgrowth happens when bacteria that normally live in the large intestine colonize the small intestine, causing bloating, gas, diarrhea, and malabsorption of nutrients. Xifaxan is one of the most commonly prescribed treatments for SIBO, though this use hasn’t received formal FDA approval.
Dosing for SIBO varies more than for the approved indications. Clinical studies have used anywhere from 200 mg to 550 mg three times daily, with treatment lasting 7 to 10 days in most cases, though some protocols extend to 28 days. The optimal dose and duration haven’t been firmly established, so your prescriber may tailor the regimen based on your symptoms and response.
Common Side Effects
Because Xifaxan stays in the gut, it tends to produce fewer side effects than antibiotics that circulate through the body. The most commonly reported side effects are digestive in nature: bloating, gas, constipation, stomach pain, and nausea. Headache, fever, and joint or muscle stiffness also show up frequently in clinical trial data.
Some people experience swelling in the hands, ankles, or feet, particularly those taking Xifaxan long-term for hepatic encephalopathy (who often have fluid retention from liver disease to begin with). Less common reports include dizziness, anxiety, trouble sleeping, and muscle spasms. Allergic reactions like hives or significant skin swelling are rare but possible.
One side effect worth noting: some people develop constipation or increased gas while taking Xifaxan for diarrhea-related conditions. This is usually temporary and resolves as the gut adjusts. The overall safety profile in long-term HE studies has been reassuring, with most patients tolerating the medication well even over months of continuous use.
What to Expect During Treatment
If you’re taking Xifaxan for IBS-D, you’re looking at a defined two-week course. Symptom improvement doesn’t always happen immediately, and the full benefit may not be apparent until toward the end of the treatment period or shortly after. If your symptoms don’t improve or they return, a second or third course is an option.
For hepatic encephalopathy, the treatment is ongoing and indefinite. The medication is working as long as you’re taking it, and stopping raises the risk of another episode. Most people on this regimen are also taking lactulose, and managing both medications is part of daily life. Around 91% of patients in the major clinical trials were using both together, which reflects standard practice.
Xifaxan is taken by mouth with or without food. Since it isn’t meaningfully absorbed into the bloodstream, drug interactions are less of a concern than with most antibiotics, though your prescriber should still know about all medications you’re taking.