Most healthy adults can take probiotics without problems, but several groups of people face real and sometimes serious risks from these live microorganisms. The people most likely to be harmed include those with weakened immune systems, critically ill patients, premature infants, and anyone with compromised gut integrity. For these groups, the very thing that makes probiotics work (living bacteria or yeast that colonize the gut) can turn dangerous if those organisms enter the bloodstream.
People With Weakened Immune Systems
This is the broadest and most well-established risk group. If your immune system is suppressed, live probiotic organisms can cross from your gut into your blood, causing serious infections. The groups at highest risk include people undergoing chemotherapy for cancer (both solid tumors and blood cancers), organ transplant recipients taking anti-rejection medications, bone marrow transplant patients, and people with advanced HIV/AIDS.
The concern isn’t theoretical. A CDC investigation found that patients using a common yeast-based probiotic had 14 times the odds of developing a fungal bloodstream infection compared to a control group. Many hospitals now advise against giving probiotics to immunocompromised patients altogether. If you’re taking immunosuppressant drugs like cyclosporine, tacrolimus, or azathioprine, probiotics could cause pathogenic colonization, meaning the “friendly” bacteria establish themselves in places they shouldn’t be and behave like an infection.
People with autoimmune conditions who take immunosuppressive therapy fall into this category too. In one documented case, a woman with rheumatoid arthritis and heart failure developed a heart valve infection caused by a Lactobacillus strain, the same type of bacteria found in many over-the-counter probiotic supplements.
Critically Ill and Hospitalized Patients
Being critically ill changes the equation dramatically. When your body is under severe physiological stress, the barriers that normally keep gut bacteria contained can break down. Patients in intensive care units, those on mechanical ventilation, or anyone with organ failure face elevated risk of probiotic organisms entering the bloodstream.
Patients with central venous catheters (IV lines placed in large veins) are at particular risk. The CDC specifically warns against yeast-based probiotics for patients who are critically ill or have indwelling catheters. In their investigation, 43% of patients who developed Saccharomyces fungemia (yeast in the blood) were actively taking that probiotic. Even proximity matters: case reports describe catheter contamination when probiotic capsules are opened near central lines.
Premature and Very Low Birthweight Infants
The FDA issued a direct warning to healthcare providers: preterm infants given probiotics are at risk of invasive, potentially fatal disease. This warning came after a premature infant weighing under 1,000 grams developed a bloodstream infection from the exact Bifidobacterium strain in the probiotic administered as part of hospital care. That infant died.
A meta-analysis covering more than 20,000 premature infants exposed to probiotics across 63 studies identified 8 cases of probiotic-caused sepsis. While that rate is low (under 0.04%), the cases clustered among the most vulnerable babies, those born extremely early (around 28 weeks) with very low birthweights. These infants have immature immune systems and fragile gut linings that allow bacteria to cross into the bloodstream far more easily than in full-term babies.
The American Academy of Pediatrics does not recommend routine probiotic use in preterm infants, particularly those born weighing under 1,000 grams. Probiotics sold as supplements are not held to pharmaceutical manufacturing standards, which means there’s no guarantee of purity, accurate labeling, or absence of contaminants.
People With Damaged or Shortened Intestines
If you have short bowel syndrome, the risk isn’t just infection. It’s a metabolic complication called D-lactic acidosis. Here’s how it works: a shortened small intestine can’t absorb carbohydrates properly, so undigested sugars reach the colon. Bacteria there, including Lactobacillus species commonly found in probiotics, ferment those carbohydrates and produce D-lactic acid. This acid builds up in the colon, and the increasingly acidic environment actually favors the growth of more acid-resistant, lactate-producing bacteria, creating a feedback loop.
When enough D-lactic acid absorbs into the bloodstream, it causes a dangerous acid buildup and neurological symptoms: confusion, slurred speech, clumsiness, unsteady walking, difficulty concentrating, and in severe cases, hallucinations and encephalopathy. Patients with higher ratios of D-lactate to L-lactate in their blood are at greater risk for these brain-related effects. Adding Lactobacillus-containing probiotics to a gut that’s already prone to bacterial overgrowth can worsen this cycle.
Anyone with other forms of compromised gut integrity, including active inflammatory bowel disease with open ulcerations, intestinal perforations, or recent gastrointestinal surgery, faces a similar underlying problem. The gut barrier that normally keeps bacteria on the inside can develop gaps that let probiotic organisms reach the bloodstream.
People With Structural Heart Conditions
This one surprises most people. If you have a prosthetic heart valve, a history of heart valve repair, or known valve abnormalities like mitral regurgitation, certain probiotic bacteria can cause endocarditis, an infection of the heart’s inner lining. Lactobacillus species are the primary concern. Case reports document endocarditis in patients with prosthetic valves, valve regurgitation, and histories of cardiac surgery who were taking Lactobacillus-containing probiotics.
Cardiac valvular disease is classified as a minor risk factor for probiotic complications, but the consequences of endocarditis are severe. It typically requires weeks of intravenous antibiotics and sometimes emergency surgery. The risk increases when heart conditions overlap with other risk factors like immunosuppression or concurrent antibiotic use.
People Taking Certain Antibiotics Alongside Probiotics
Taking probiotics during a course of antibiotics is one of the most common reasons people start them in the first place, usually to prevent diarrhea. For most healthy people, this is reasonable. But there’s a specific scenario where it backfires: when the antibiotic you’re taking happens to be one the probiotic strain is resistant to. In that case, the antibiotic kills off your normal gut bacteria while leaving the probiotic strain free to overgrow without competition. In someone who already has a compromised gut barrier or weakened immunity, this unopposed colonization can tip into an actual infection.
What the Risk Comes Down To
The common thread across all these groups is the same: a weakened barrier (whether it’s the immune system, the gut wall, or a damaged heart valve) that can’t keep probiotic organisms where they belong. In a healthy person, even if probiotic bacteria occasionally cross the gut lining, the immune system clears them without incident. In vulnerable populations, those organisms can establish infections in the blood, heart, or other organs.
If you’re generally healthy and not in any of the groups above, probiotics carry minimal risk, though their benefits for most conditions are more modest than marketing suggests. If you do fall into one of these categories, the decision to use probiotics should involve weighing specific, documented harms against whatever benefit you’re hoping to get. For the most vulnerable, like extremely premature infants or patients in the ICU, the answer is straightforward: the risk isn’t worth it.