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Who should not take probiotics? What the research says

By Published 6 min read

Probiotics are live bacteria or yeast. That is exactly the point: they have to be alive to work. But it also means that in rare cases they can end up somewhere they do not belong.

For the vast majority of people, this is nothing to worry about. The question is who it applies to, and who should talk to their doctor first.

For healthy people: no more side effects than placebo

The large reviews of the trials point the same way:

  • A meta-analysis of probiotics for irritable bowel syndrome pooled side-effect data from 55 trials with more than 7,000 patients. The risk of side effects was not significantly higher with probiotics than with placebo [1].
  • A Cochrane review of probiotics for colds also found no difference in side effects (8 trials, 2,456 participants). The side effects that did occur were minor and mostly from the stomach: wind, vomiting, diarrhoea and stomach pain [2].
  • A meta-analysis of 36 trials with 9,312 adults who were taking antibiotics also found no more side effects with probiotics than with placebo [3].

So a little wind and an unsettled stomach in the first few days is the most common thing. It is unpleasant, but not dangerous.

When the bacteria end up in the blood

For the sickest patients, the picture looks different. Here there are several examples of the bacterium or yeast from the supplement being found in the blood.

Intensive care patients. In a large trial with 2,653 patients on ventilators in 44 intensive care units, half were given Lactobacillus rhamnosus GG. It did not prevent pneumonia, nor did it help with any of the 20 other things the researchers measured. On the other hand, the bacterium was found in the blood or other places where it does not belong in 15 patients, against one in the placebo group [4]. The researchers' conclusion: the findings do not support giving L. rhamnosus GG to critically ill patients.

The same strain as in the capsule. At a children's hospital in Boston, 6 out of 522 intensive care patients who were given probiotics got Lactobacillus in their blood. Among the more than 21,000 patients who were not given them, it was only 2. When the researchers examined the bacteria's genetic material, they could not tell the bacteria in the blood apart from those in the capsules. In one patient the bacterium had also developed resistance to an antibiotic [5].

The yeast S. boulardii. The yeast Saccharomyces boulardii is one of the best-studied probiotics for diarrhoea. At five Finnish university hospitals, 46 patients were found with Saccharomyces yeast in their blood over ten years. At least 20 of them were taking S. boulardii as a supplement [6]. An American study of more than 16,000 inpatients who were given S. boulardii found yeast in the blood in 18, or about 1 in 900. The risk was highest in intensive care, and the authors judged that it was no greater than the risk of other bloodstream infections in hospital [7].

Premature babies. Many neonatal units give premature babies probiotics to prevent a serious bowel disease. At an English hospital, after ten years of routine use, a bifidobacterium from the product was found for the first time in a baby's blood. The authors call it rare [8]. The point is that the decision about probiotics for premature babies belongs with the doctors, not on the pharmacy shelf.

The trial where more people died

The most serious result comes from a Dutch trial with 298 patients with severe acute inflammation of the pancreas. Half were given a mixture of several probiotic strains through a feeding tube for up to four weeks, to prevent infections.

It did not. Infections affected 30% with probiotics and 28% with placebo. But 24 patients died in the probiotic group against 9 in the placebo group, and 9 in the probiotic group developed a lack of oxygen in the bowel, against none with placebo. Eight of them died [9].

The trial is about very sick patients and a specific product given through a tube, not about a healthy person taking a capsule. But it shows that probiotics are not automatically harmless just because they are sold as a food supplement.

What about "brain fog"?

You may come across the claim that probiotics cause brain fog. It comes from a small American study that described 30 patients with brain fog, wind and bloating. All of them were taking probiotics, and many had bacterial overgrowth in the small intestine and too much lactic acid in the body. When they stopped the probiotics and took a course of antibiotics, most of them felt better [10].

The study had no randomisation, only 8 people in the comparison group, and the participants received two treatments at once. So it does not prove that probiotics were the cause. It is a hypothesis, not a finding. But if a supplement makes you feel worse, that is a good argument for stopping and seeing whether it passes.

Talk to your doctor first if you

  • are seriously ill or in hospital, especially in intensive care
  • have a catheter in a vein, e.g. a central venous catheter
  • have a weakened immune system, e.g. after an organ transplant, during chemotherapy or for other reasons
  • have acute inflammation of the pancreas
  • have a baby who was born prematurely

The Cochrane review of probiotics for C. difficile found short-term use safe in patients who were not immunocompromised or severely debilitated [11]. That is exactly the line: for healthy people the risk is very small. For the sickest it is real, and a benefit has often not been shown.

What you can do

  • If you are healthy, probiotics are very likely harmless. That does not mean they work. Read about what it takes in the article on CFU and bacterial strains.
  • A little wind at the start is common. If you feel clearly worse, stop.
  • Tell your doctor what you take if you are admitted to hospital. Supplements are easily forgotten when you list your medicines, but live bacteria and yeast can matter when you are ill.
  • For children, and especially for premature babies, ask your doctor or health visitor first.

The bottom line

  • For healthy adults, the trials show no more side effects with probiotics than with placebo. A little wind is the most common.
  • In intensive care patients, inpatients and premature babies, probiotic bacteria and yeast have been found in the blood, and in several cases it was the same strain as in the supplement.
  • In a trial in severe pancreatitis, more people died with probiotics than with placebo.
  • If you are seriously ill or have a weakened immune system, talk to your doctor before taking probiotics.

Sources

  1. Goodoory VC et al. Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis. Gastroenterology. 2023. PMID 37541528
  2. Zhao Y, Dong BR, Hao Q. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. 2022. PMID 36001877
  3. Liao W et al. Probiotics for the Prevention of Antibiotic-associated Diarrhea in Adults: A Meta-Analysis of Randomized Placebo-Controlled Trials. J Clin Gastroenterol. 2021. PMID 33234881
  4. Johnstone J et al. Effect of Probiotics on Incident Ventilator-Associated Pneumonia in Critically Ill Patients: A Randomized Clinical Trial. JAMA. 2021. PMID 34546300
  5. Yelin I et al. Genomic and epidemiological evidence of bacterial transmission from probiotic capsule to blood in ICU patients. Nat Med. 2019. PMID 31700189
  6. Rannikko J et al. Fungemia and Other Fungal Infections Associated with Use of Saccharomyces boulardii Probiotic Supplements. Emerg Infect Dis. 2021. PMID 34287140
  7. Wombwell E, Bransteitter B, Gillen LR. Incidence of Saccharomyces cerevisiae fungemia in hospitalised patients administered Saccharomyces boulardii probiotic. Mycoses. 2021. PMID 34585799
  8. Acuna-Gonzalez A et al. Bifidobacterium bacteraemia is rare with routine probiotics use in preterm infants: A further case report with literature review. Anaerobe. 2023. PMID 36754140
  9. Besselink MG et al. Probiotic prophylaxis in predicted severe acute pancreatitis: a randomised, double-blind, placebo-controlled trial. Lancet. 2008. PMID 18279948
  10. Rao SSC et al. Brain fogginess, gas and bloating: a link between SIBO, probiotics and metabolic acidosis. Clin Transl Gastroenterol. 2018. PMID 29915215
  11. Goldenberg JZ et al. Probiotics for the prevention of Clostridium difficile-associated diarrhea in adults and children. Cochrane Database Syst Rev. 2017. PMID 29257353

This article is general information and not medical advice.