A randomized controlled trial of 20 healthy volunteers found that taking a synbiotic gel containing Lactobacillus acidophilus and Bifidobacterium lactis during rifaximin antibiotic treatment did not significantly increase beneficial bacteria levels compared to placebo. While traces of the probiotic bacteria appeared in some stool samples, suggesting temporary passage through the digestive system, the bacteria did not establish lasting populations. This research suggests that this particular probiotic formulation may not provide the protective benefits against antibiotic-related gut bacteria loss that many people expect.
Researchers wanted to know if taking a special probiotic gel alongside rifaximin (an antibiotic used for digestive problems) could protect the good bacteria in your gut. They gave 20 healthy people either the probiotic gel or a placebo while they took the antibiotic for one week, then tracked their gut bacteria for four more weeks. According to Gram Research analysis, the probiotic gel didn’t significantly increase the good bacteria counts in either group, though some people did show traces of the probiotic bacteria in their stool samples. This suggests that while the probiotics may have temporarily passed through the digestive system, they didn’t establish lasting colonies in the gut.
Key Statistics
A randomized controlled trial published in 2026 with 20 healthy volunteers found that synbiotic gel containing 4 billion CFU each of Lactobacillus acidophilus and Bifidobacterium lactis did not significantly increase beneficial bacteria counts during or after rifaximin antibiotic treatment compared to placebo.
According to research reviewed by Gram, traces of the specific probiotic bacteria appeared in some participants’ stool samples, indicating transient passage through the digestive system rather than permanent colonization, suggesting the bacteria did not establish lasting populations in the gut.
The study detected Lactobacillus acidophilus NCFM and Bifidobacterium lactis Bi07 in only some stool samples from the probiotic group, highlighting that even high-dose probiotics (4 × 10⁹ CFU per strain daily) may not reliably establish themselves in the human gut during antibiotic treatment.
The Quick Take
- What they studied: Whether taking a probiotic gel containing beneficial bacteria (Lactobacillus and Bifidobacterium) alongside rifaximin antibiotic treatment could protect or restore healthy gut bacteria in people without existing digestive problems.
- Who participated: 20 healthy adult volunteers who hadn’t taken any antibiotics in the previous month. They were randomly split into two groups of 10—one received the probiotic gel while taking rifaximin, and the other received a placebo gel with the same antibiotic.
- Key finding: After five weeks of taking the probiotic gel during and after rifaximin treatment, the levels of beneficial bacteria (lactobacilli and bifidobacteria) in participants’ stool samples did not significantly increase compared to the placebo group. However, some participants showed temporary traces of the probiotic bacteria, suggesting the bacteria passed through but didn’t permanently settle.
- What it means for you: Taking this particular probiotic product alongside antibiotics may not be as effective at protecting your gut bacteria as previously hoped. The bacteria appear to pass through your system temporarily rather than establishing lasting colonies. More research with higher doses or different formulations may be needed to find truly effective probiotic strategies during antibiotic treatment.
The Research Details
This was a randomized controlled trial, which is one of the strongest types of scientific studies. Researchers divided 20 healthy volunteers into two equal groups without telling them which group received the real probiotic gel and which received a fake version (placebo). This “blinding” prevents bias from affecting the results. One group took a synbiotic gel (a combination of probiotics and prebiotics—food that feeds good bacteria) containing Lactobacillus acidophilus and Bifidobacterium lactis along with agave inulin as a prebiotic, while also taking rifaximin antibiotic for the first week. The other group took the same antibiotic but received a placebo gel instead. Researchers collected and analyzed stool samples to count the actual numbers of beneficial bacteria colonies at different time points throughout the five-week study period.
This research design is important because it directly measures what actually happens to gut bacteria during and after antibiotic treatment when probiotics are added. Rather than relying on theory or test-tube studies, researchers counted the actual bacteria in real human stool samples. This gives us concrete data about whether probiotics truly colonize the gut or just pass through temporarily. The randomized, placebo-controlled design eliminates the possibility that people’s expectations or lifestyle changes (rather than the probiotic itself) caused any observed effects.
This study has several strengths: it used a randomized, double-blind, placebo-controlled design (the gold standard for clinical trials), measured actual bacterial counts rather than relying on self-reported symptoms, and tracked participants for a full five weeks after treatment ended. However, the study is limited by its very small sample size (only 10 people per group), which makes it harder to detect real effects and easier for random variation to influence results. The study was conducted in healthy volunteers without digestive problems, so results may not apply to people with existing gut issues or those taking antibiotics for infections. Additionally, the study only tested one specific probiotic product at one specific dose, so results don’t necessarily apply to other probiotic formulations.
What the Results Show
The main finding was surprising: after five weeks of taking the synbiotic gel (which contained billions of beneficial bacteria per dose) alongside rifaximin and for four weeks after, the levels of lactobacilli and bifidobacteria in stool samples did not significantly increase in the probiotic group compared to the placebo group. In other words, the probiotic gel did not appear to boost or protect the good bacteria populations in the gut. This was unexpected because previous research suggested that probiotics should help maintain healthy bacteria during antibiotic treatment.
However, the researchers did find something interesting: some participants in the probiotic group showed traces of the specific probiotic bacteria (Lactobacillus acidophilus NCFM and Bifidobacterium lactis Bi07) in their stool samples. This indicates that the bacteria from the gel did reach the intestines and were detectable in the stool. The fact that these bacteria appeared only in some samples and didn’t establish permanent populations suggests they were passing through the digestive system rather than taking up residence and multiplying.
The placebo group, as expected, showed no traces of these specific probiotic strains. Both groups experienced similar changes in their overall gut bacteria populations during and after rifaximin treatment, suggesting that the probiotic gel provided no additional protection compared to taking the antibiotic alone.
The researchers noted that the prebiotic component (agave inulin) in the synbiotic gel was chosen because previous laboratory studies showed it was superior to other prebiotics like chicory inulin at feeding beneficial bacteria. However, this advantage in the lab didn’t translate to measurable benefits in actual human guts. The study also highlighted that even though participants received a substantial dose of probiotics (4 billion colony-forming units of each bacterial strain per day), this wasn’t enough to establish lasting populations. This suggests that simply consuming probiotics may not be sufficient—the bacteria need to be able to survive the journey through stomach acid, bile, and the competitive environment of the existing gut microbiota.
This research fills an important gap in the scientific literature. While many studies have examined probiotics alone or antibiotics alone, very few have directly measured actual bacterial counts in healthy people taking both together. Previous research suggested that probiotics should help prevent dysbiosis (an imbalance of gut bacteria) during antibiotic treatment, but this study provides the first direct evidence that this particular probiotic product may not be as effective as hoped. The findings align with growing recognition in the scientific community that not all probiotics work equally well, and that the ability of probiotic bacteria to actually colonize and establish themselves in the human gut is more limited than once believed.
The most significant limitation is the very small sample size—only 10 people per group. With such small numbers, it’s difficult to detect real effects, and random variation can heavily influence results. The study only included healthy volunteers without existing digestive problems, so we don’t know if results would be different in people with gut disorders or those taking antibiotics to treat infections. The research only tested one specific probiotic product at one specific dose, so we can’t conclude that all probiotics are ineffective—just this particular formulation. The study lasted only five weeks, which may not be long enough to see lasting changes in gut bacteria populations. Additionally, the researchers didn’t measure other important factors like participants’ diets, stress levels, or sleep patterns, which could influence gut bacteria independently of the probiotic treatment.
The Bottom Line
Based on this research, taking this specific synbiotic gel during rifaximin treatment does not appear to provide measurable protection for beneficial gut bacteria in healthy people (low confidence for this specific product). However, this doesn’t mean all probiotics are ineffective—this study tested only one formulation. If you’re taking rifaximin or another antibiotic, discuss probiotic options with your doctor. They may recommend waiting until after antibiotic treatment ends before starting probiotics, or they may suggest different probiotic strains that have stronger evidence. Eating fiber-rich foods (which naturally feed good bacteria) during and after antibiotic treatment may be a more reliable strategy than probiotics alone.
This research is most relevant to people considering taking probiotics during antibiotic treatment, particularly those taking rifaximin for digestive disorders. Healthcare providers prescribing antibiotics should be aware that this particular probiotic product may not provide the protective benefits previously assumed. However, the findings shouldn’t discourage people from exploring other probiotic options or from maintaining good gut health through diet and lifestyle. People with existing digestive problems or those taking antibiotics for infections should consult their healthcare provider before making decisions based on this study, since the results come from healthy volunteers without gut issues.
Based on this study, if you were to take this probiotic product, you shouldn’t expect to see measurable increases in beneficial bacteria within the five-week study period. The bacteria appear to pass through the system within days rather than establishing lasting populations. If you’re considering probiotics for gut health, realistic expectations are that benefits (if any) would likely take several weeks to months to become noticeable, and would depend heavily on the specific probiotic strain, dose, and your individual gut environment.
Frequently Asked Questions
Do probiotics actually work when you’re taking antibiotics?
This study found that one specific probiotic gel did not significantly protect beneficial bacteria during antibiotic treatment in healthy people. However, results vary by probiotic strain and dose. Consult your doctor about which probiotics, if any, might help your specific situation.
Should I take probiotics while taking rifaximin?
This research suggests this particular synbiotic product may not provide measurable benefits during rifaximin treatment. Talk to your prescribing doctor about whether probiotics are recommended for your specific condition and which products have evidence supporting their use.
How long does it take for probiotics to work in your gut?
This study tracked participants for five weeks and found no significant increase in beneficial bacteria. The probiotic bacteria appeared temporarily in some stool samples but didn’t establish lasting populations, suggesting they passed through rather than colonizing the gut.
Can probiotics prevent dysbiosis from antibiotics?
This study found that taking a synbiotic gel during antibiotic treatment did not prevent changes in beneficial bacteria populations compared to placebo. More research with different probiotic formulations and doses is needed to determine which, if any, can effectively prevent dysbiosis.
What’s the difference between probiotics and synbiotics?
Synbiotics combine probiotics (live beneficial bacteria) with prebiotics (food that feeds those bacteria). This study tested a synbiotic containing Lactobacillus, Bifidobacterium, and agave inulin, but even this combination didn’t establish lasting bacterial populations during antibiotic treatment.
Want to Apply This Research?
- Track daily probiotic intake (yes/no) and antibiotic doses alongside weekly digestive symptoms (bloating, gas, bowel regularity on a 1-10 scale) to monitor whether probiotics correlate with symptom changes, even if bacterial counts don’t increase.
- Instead of relying solely on probiotic supplements during antibiotic treatment, increase fiber intake through foods like oats, beans, and vegetables to naturally feed whatever beneficial bacteria remain. Log daily fiber grams and digestive symptoms to track correlation.
- Create a 6-week tracking protocol: record probiotic product name and dose daily, note antibiotic schedule, and rate digestive comfort daily. After treatment ends, continue tracking for 4 weeks to observe whether symptoms improve, decline, or remain stable—providing personal data on whether this specific probiotic helped your individual gut.
This research describes findings from a small study in healthy volunteers and should not be interpreted as medical advice. The results apply specifically to the synbiotic gel tested and may not generalize to other probiotic products, doses, or formulations. If you are taking rifaximin or any antibiotic, consult your healthcare provider before starting probiotics or making changes to your treatment plan. Individual responses to probiotics vary significantly, and what doesn’t work for one person may work differently for another. This study does not replace professional medical guidance for managing antibiotic side effects or maintaining gut health.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.