Reuteri Probiotic: Benefits, Dosage, and Side Effects

Lactobacillus reuteri (now formally reclassified as Limosilactobacillus reuteri) is one of the most extensively studied probiotic species, with clinical evidence supporting benefits that range from reducing infant colic to lowering LDL cholesterol. But the effects depend heavily on which strain you take and what you’re taking it for, and many of the most eye-catching findings come from animal research that hasn’t yet been replicated in humans. Understanding what the evidence actually shows, where it’s strong, and where it’s still thin can help you decide whether this probiotic is worth adding to your routine.

How L. Reuteri Works in the Body

L. reuteri is a natural resident of the human gut, though modern populations tend to carry it at lower levels than their ancestors likely did. One of its distinctive traits is the ability to convert glycerol into reuterin, a broad-spectrum antimicrobial compound that inhibits both gram-positive and gram-negative bacteria.¹ Reuterin works by disrupting thiol groups on proteins, which effectively damages a large number of cellular targets in harmful bacteria.² This gives L. reuteri a competitive edge in the gut, allowing it to suppress pathogens while carving out its own niche.

Beyond reuterin production, L. reuteri strains influence the immune system through several pathways. The bacterium has been shown to regulate intestinal microbiota composition, interact with immune cells, modulate cytokine production, and influence tryptophan metabolism and antioxidant enzyme activity.³ These mechanisms vary by strain, which is why different L. reuteri strains are used for different health conditions.

Infant Colic

The strongest and most consistent clinical evidence for L. reuteri involves infant colic. The strain DSM 17938 has been tested in multiple trials and consistently reduces crying time in colicky, breastfed infants. In one randomized controlled trial, daily crying time in the probiotic group fell from roughly 200 minutes per day to about 32 minutes, compared to about 120 minutes in the placebo group.⁴ A real-world clinic study found that about 85% of infants given this strain achieved at least a 50% reduction in crying time over the study period, with mean crying dropping from around 248 minutes at baseline to about 46 minutes.⁵

Both studies also noted improvements in maternal depression scores alongside the reduction in infant crying, which makes intuitive sense: a baby who screams for four hours a day is exhausting for everyone. The colic evidence is strong enough that some pediatric guidelines now mention L. reuteri DSM 17938 as a consideration for breastfed colicky infants, though the data in formula-fed infants is less consistent.

Cholesterol Reduction

A different strain, L. reuteri NCIMB 30242, has shown meaningful effects on blood lipids in adults with high cholesterol. This strain produces bile salt hydrolase, an enzyme that breaks apart conjugated bile acids in the gut. When those bile acids are deconjugated, the body pulls cholesterol from the bloodstream to make new ones, effectively lowering circulating levels. In a randomized trial using capsules, LDL cholesterol dropped by about 12% relative to placebo, total cholesterol by about 9%, and apoB-100 by roughly 8%.⁶

A separate trial using the same strain in yogurt form found similar, though slightly smaller, reductions: about 9% for LDL and roughly 5% for total cholesterol.⁷ The mechanism appears to involve reduced absorption of non-cholesterol sterols like campesterol and sitosterol, which dropped by 35 to 42% in treated participants.⁸ These are not small effects for a probiotic, but they have been studied primarily in adults who already had elevated cholesterol. If your levels are normal, there is no reason to expect the same magnitude of change.

Gut Barrier and Intestinal Health

Your intestinal lining is held together by tight junction proteins that control what passes through the gut wall and what stays out. When this barrier breaks down, bacteria and toxins can leak into the bloodstream and trigger inflammation. L. reuteri has shown the ability to strengthen this barrier in both lab and animal studies. In newborn piglets given L. reuteri I5007 for two weeks, the levels of key tight junction proteins like claudin-1, occludin, and ZO-1 all increased significantly.⁹

In mouse models of intestinal infection, L. reuteri protected the mucosal barrier by modulating the Wnt/β-catenin signaling pathway, maintaining stem cell populations in the gut lining, and stimulating epithelial cell growth to repair damaged tissue. It also reduced inflammatory cytokine levels and lowered bacterial toxins in the bloodstream.¹⁰ These findings are consistent with broader research showing that certain Lactobacillus strains, along with other probiotic species, can upregulate tight junction proteins and suppress inflammatory signaling in the gut.¹¹ The translational gap here is worth noting: most of this evidence comes from cell cultures and animal models, and the direct clinical relevance for humans with leaky gut complaints is still being worked out.

Oral Health

L. reuteri has gained attention in dentistry, particularly for its effects on gum disease. In a clinical trial involving periodontitis patients, those receiving L. reuteri alongside standard treatment saw gum pocket depth improve by about 36%, with roughly two-thirds achieving a clinically meaningful reduction of at least 2 millimeters, compared to only about 13% in the placebo group. Inflammatory markers like TNF-α and IL-6 each dropped by about 45%, and gingival pain decreased by nearly half.¹² Radiographic bone loss also improved, with about two-thirds of test patients showing improvement to milder grades of bone loss.

The antimicrobial properties of L. reuteri are thought to help shift the oral microbiome away from the pathogenic bacteria that drive gum disease.¹³ Oral probiotic products containing L. reuteri typically come as lozenges or chewable tablets designed to be dissolved slowly in the mouth rather than swallowed, which makes sense: you need the bacteria to colonize the oral cavity, not just pass through on the way to the intestines.

Bone Density

The bone health evidence is a good example of promising early results that didn’t hold up in a larger follow-up. An initial trial in older women with low bone mineral density found that L. reuteri 6475 cut the rate of bone loss roughly in half compared to placebo over one year. The treated group lost about 0.83% of total volumetric bone density, while the placebo group lost about 1.85%.¹⁴

However, a larger randomized trial of 239 early postmenopausal women found that L. reuteri 6475 had no effect on bone loss or bone turnover over two years.¹⁵ The different populations matter here: the first trial enrolled women who already had low bone density, while the second looked at women in early menopause before severe bone loss had set in. The upshot is that L. reuteri cannot be recommended for bone health at this point, but the question isn’t completely closed for women who already have measurable bone loss.

The Histamine Question

One aspect of L. reuteri that catches people off guard is that certain strains produce histamine. L. reuteri 6475 carries a histidine decarboxylase gene cluster that converts the amino acid histidine into histamine in the gut.¹⁶ In the body, this histamine activates the H2 receptor, which paradoxically suppresses inflammation rather than triggering the allergic-type symptoms most people associate with histamine. In mouse models, this H2 receptor signaling was necessary for the strain’s ability to suppress colitis and reduce inflammatory cytokines in the colon.¹⁷

This is where it gets complicated for people with histamine intolerance or mast cell activation syndrome. The anti-inflammatory benefits of bacterially produced histamine depend on H2 receptor signaling, but if you’re someone whose body already struggles to break down histamine, adding more from a bacterial source could theoretically worsen symptoms like headaches, flushing, or digestive upset. Not all L. reuteri strains carry the hdc gene cluster, though, so this concern is strain-specific. DSM 17938, the infant colic strain, does not produce histamine the same way 6475 does. If histamine sensitivity is a concern for you, checking which strain a product contains matters more than knowing it’s “L. reuteri.”

Social Behavior and the Gut-Brain Axis

Some of the most fascinating L. reuteri research involves its effects on social behavior in mouse models of autism spectrum disorder. Researchers found that L. reuteri reversed social deficits in several genetic and environmental mouse models for neurodevelopmental disorders. The effect depended on the vagus nerve: when the vagus nerve was severed, L. reuteri no longer rescued social behavior. The bacterium restored social interaction-induced synaptic plasticity in the ventral tegmental area of the brain, and this effect required functioning oxytocin receptors.¹⁸ Further work has confirmed the involvement of oxytocin and biopterin pathways.¹⁹

This is genuinely compelling neuroscience, but it’s important to be clear: these are mouse studies. No controlled human trial has demonstrated that L. reuteri supplements improve social behavior in people with autism or any other neurodevelopmental condition. The research establishes a biological mechanism worth investigating, not a treatment recommendation. Anyone marketing L. reuteri as a brain supplement or autism therapy is outrunning the evidence.

Skin and Hair

The “glow of health” studies are among the most widely shared L. reuteri findings online, and they are real research from a reputable lab at MIT. In aged mice, daily consumption of L. reuteri led to thicker, more lustrous fur, more active hair follicles, increased skin thickness, and more acidic skin pH mimicking youthful conditions. The mechanism depended on interleukin-10, an anti-inflammatory cytokine.²⁰ A related study from the same group found that probiotic-fed mice maintained youthful reproductive fitness, including larger testes and higher testosterone levels, mediated by oxytocin signaling.²¹

For human skin specifically, a proof-of-concept trial tested topical L. reuteri DSM 17938 in adults with atopic dermatitis. The probiotic product reduced the SCORAD severity index by about 46% over eight weeks, though the control product also showed a reduction of about 39%, and the difference between groups was not statistically significant.²² Lab studies suggest that both live L. reuteri and its lysate (broken-down cell fragments) can reduce UV-induced inflammatory markers and enhance skin barrier proteins like aquaporin 3.²³ The hair and skin benefits remain squarely in the promising-but-preliminary category for humans.

Dosage and How Long It Sticks Around

Most clinical trials use doses between 100 million and 10 billion colony-forming units (CFU) per day, with the most common dose for well-studied strains like DSM 17938 being around 100 million to 1 billion CFU daily. In a colonization study, healthy adults consuming about 1 billion CFU of L. reuteri DSM 17938 daily for three weeks showed significantly increased levels of viable L. reuteri in their stool compared to baseline.²⁴

L. reuteri does not permanently colonize the gut in most people, however. In a study comparing daily versus alternate-day supplementation over a week, fecal L. reuteri counts rose significantly in both groups during the dosing period, but levels dropped within one to two weeks after supplementation stopped. Only about two out of nine volunteers in each group still had detectable L. reuteri two weeks later.²⁵ This means you generally need to keep taking it to keep getting the effects. Interestingly, alternate-day dosing initially produced higher fecal counts than daily dosing in the first few days, though the groups evened out by the end of the week and had similar persistence afterward.

Safety and Side Effects

L. reuteri has a strong safety profile across dozens of clinical trials. In a randomized, masked trial in healthy adults, there were no serious adverse events, and the rate of everyday symptoms like mild digestive discomfort was identical between the probiotic and placebo groups.²⁶ Even in immunocompromised individuals, safety data is reassuring: a study in HIV-positive people found no clinically significant changes in safety parameters at doses up to 10 billion CFU per day.²⁷

The most commonly reported side effects across trials are mild and transient: occasional gas, bloating, or loose stools in the first few days of use. These tend to resolve on their own as the gut microbiome adjusts. For people taking anticoagulants, one theoretical concern with the cholesterol-lowering strain NCIMB 30242 involves its bile salt hydrolase activity potentially affecting the absorption of fat-soluble vitamins, though this hasn’t been flagged as a clinical issue in trials.

Standard precautions apply for anyone who is severely immunocompromised (such as transplant recipients on heavy immunosuppression or people with central venous catheters), where any live bacterium poses a small but non-zero risk of translocation into the bloodstream. If you fall into that category, talk to your doctor before starting any probiotic, not just L. reuteri.

Why the Strain Name on the Label Matters

A recurring theme across all of this research is that L. reuteri is not one thing. The species contains remarkable genetic diversity, with recent genomic analysis identifying at least eight distinct lineages beyond the six subspecies already described, each adapted to different vertebrate hosts.²⁸ The functional differences between strains are not subtle: DSM 17938 reduces infant colic, NCIMB 30242 lowers cholesterol, and 6475 produces histamine that suppresses inflammation through H2 receptors. A product that simply lists “L. reuteri” without specifying the strain is essentially telling you it contains some member of a very large family, without telling you which one.

When shopping for an L. reuteri supplement, look for the full strain designation on the label (the letters and numbers after the species name). If a product doesn’t list the strain, you have no way to match it to any of the research described here. Encapsulation and delivery format also affect whether live bacteria survive stomach acid and reach the intestines. Maintaining viability during manufacturing, storage, and transit through the gastrointestinal tract remains one of the practical challenges with L. reuteri products, and the technology to address this, including microencapsulation techniques, is still evolving.

What the Animal Studies Suggest but Haven’t Proven

A lot of the excitement around L. reuteri comes from animal research that is genuinely interesting but hasn’t been confirmed in people. The MIT group’s findings on thicker fur, increased testosterone, and youthful reproductive fitness in aged mice made headlines, and the mechanism they identified, involving anti-inflammatory IL-10 and the neuropeptide oxytocin, is biologically coherent.²⁹ Similarly, the gut-brain axis work showing L. reuteri rescuing social behavior via vagus nerve and oxytocin pathways is a compelling line of inquiry. But mouse physiology is not human physiology, and the history of medicine is littered with promising mouse results that failed in human trials.

The most reliable approach is to treat the well-replicated human trial data, like the infant colic and cholesterol results, as established benefits, and to regard the animal model findings as hypotheses that might pan out. If you’re taking L. reuteri hoping for thicker hair or higher testosterone based on mouse studies, you’re making a bet on incomplete information. That’s your call to make, but it should be an informed one.