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What is a probiotic?

A product is considered to be probiotic if it contains microorganisms that provide benefit when ingested. There are numerous types of microorganism that can be found in probiotic products, e.g. Lactobacillus acidophilus, L. rhamnosus and Bifidobacterium lactis (bacteria used in the production of dairy products) and Saccharomyces boulardii (a strain of yeast). Supplements containing freeze-dried microorganisms, marketed as being probiotic, are available over-the-counter (OTC) at pharmacies, “health” stores and supermarkets. Live cultures of microorganisms are also sometimes added to commercially produced foods, e.g. probiotic yoghurt. However, whether or not probiotic products actually contain live microorganisms at the time of consumption, and even assuming they do, it is largely unknown whether any of these microorganisms are able to form beneficial living colonies in the person taking them.

The majority of the potential health benefits attributed to probiotics are thought to occur in the host’s gastrointestinal tract. These may include: improved digestion and nutrient absorption, enhanced immune function and re-colonisation of the gastrointestinal tract following a disturbance in gut microflora, as can occur after treatment with antibiotics.

The concentration of microorganisms in a probiotic is generally reported in colony forming units (CFU). Patients who are taking antibiotics are reported to need to ingest between five and 40 billion CFU/day of a probiotic organism in order to experience any benefit.1

Do probiotics protect against antibiotic-associated diarrhoea?

When ingested in sufficient quantities, probiotics do appear to reduce the risk of antibiotic-associated diarrhoea in children and adults. This protective ability appears to broadly extend across different types of antibiotic and different probiotics.

A meta-analysis of 16 studies, including over 3400 young people aged between two weeks and 17 years, reported a number needed to treat (NNT) of seven patients for the probiotics Lactobacillus rhamnosus or Saccharomyces boulardii to prevent one case of antibiotic-associated diarrhoea.1 Another meta-analysis of 34 randomised controlled trials that included over 4100 patients assessed regimens of: bifidobacteria, lactobacilli, enterococci, streptococci, Saccharomyces boulardii and combinations of these probiotics.5 The NNT for the probiotics to prevent one episode of antibiotic-associated diarrhoea was eight patients.5 Probiotics were most effective at preventing diarrhoea in patients taking antibiotics for H. pylori with a NNT of five.5

The duration of probiotic treatment varied between the studies, but a broadly representative regimen would be commencing probiotics on the day that antibiotic treatment was begun, and continuing for at least as long as the course of antibiotic treatment and up to one week after.5 The authors concluded that there was a relatively consistent protective effect across the range of probiotics studied, that this effect was seen in paediatric and adult populations and was maintained across different antibiotics and different indications for the antibiotic use.5

Antibiotic use is associated with a range of gastrointestinal adverse effects

By altering the gut microflora, antibiotics can cause a range of adverse effects, including Clostridium difficile colitis and antibiotic-associated diarrhoea. Diarrhoea is reported to occur in 5 – 30% of patients prescribed antibiotics,2 and is most likely to occur with broad spectrum antibiotics, e.g. penicillin derivatives, cephalosporins and clindamycin.3 The use of antibiotics is also associated with approximately a four-fold increased risk of vaginal candidiasis in females.4 Some patients who are prescribed antibiotics will consider taking adjunctive probiotics in order to reduce their risk of experiencing antibiotic-associated adverse effects.

Do probiotics protect against vaginal candidiasis?

In contrast to their effectiveness in preventing antibiotic-associated diarrhoea, there is evidence that probiotics are not protective against post-antibiotic vulvovaginal candidiasis. A study of 235 non-pregnant females aged 18 – 50 years found that preparations of Lactobacillus taken orally or vaginally, or by both routes, did not reduce the risk of developing post-antibiotic vulvovaginal candidiasis.6

But are probiotics safe?

The issue of probiotic safety is complicated by two factors. Firstly, adverse effects are frequently poorly reported in studies, as the assumption is often made that probiotics are safe. A meta-analysis of trials that did include adverse events found no statistical difference in the incidence of symptoms such as rash, vomiting or other gastrointestinal effects and cough between treatment and control groups.1 Secondly, the range of bacteria that are included in probiotic products is vast. It is therefore not possible to comment conclusively on the safety of all probiotics. This makes it very difficult to balance the risks and the benefits of probiotics.

A review focusing on Lactobacilli discussed three theoretical safety concerns associated with the use of probiotics:7

  1. The possibility of infectious disease
  2. Toxicity in the gastrointestinal tract
  3. The possible transfer of antibiotic resistance from probiotic bacteria to microflora in the gastrointestinal tract

Lactobacillus bacteria species that are present in probiotics can cause bacteraemia and endocarditis; immunosuppressed people appear to be at the greatest risk.7 Although rare, there have been a number of cases of bacteraemia or fungemia related to the use of probiotics.7 However, epidemiological data does not indicate any increase in these infections associated with the use of probiotics.7 Based on the limited evidence available, the risk of infections related to probiotic use is very small and appears to be higher in young children with short gut syndrome and older patients who are immunosuppressed, e.g. following HIV infection, or those who have had an organ transplant.7

There are a range of toxic effects that probiotics could theoretically cause in the gastrointestinal tract, including: malabsorption due to disruption of bile salts, lactate production resulting in lactic acidosis, modification of immune responses, degradation of intestinal mucus.7 However, there is little evidence suggesting that any of these theoretical effects are likely to be clinically relevant to patients who do not have underlying gastrointestinal abnormalities, e.g. short gut syndrome, although in these patients the consequences could be serious.7

Bacteria within the human intestine are able to share genes that confer antibiotic resistance between themselves and they are also able to acquire or donate these resistance genes to other bacteria within the gastrointestinal tract.8 Some probiotic lactic acid bacteria have been isolated that are resistant to certain antibiotics, e.g. erythromycin, vancomycin, tetracycline, and probiotic bacteria have also been shown to be able to transfer antibiotic resistance to other species of bacteria in vitro.8 This has lead to a safety concern that because cultures used in the production of probiotics are not routinely screened for antibiotic resistance, it may be possible for potentially pathogenic bacteria within the gastrointestinal tract to acquire antibiotic resistance from probiotic bacteria.8 However, as is the case with many aspects of probiotics, it is currently unknown how likely this possibility is.

More extensive research, and consistent reporting of adverse reactions via pharmacovigilance monitoring programmes, would be expected to provide better information on the likelihood, nature and seriousness of adverse events with probiotics.


Thank you to Eamon Duffy, Antimicrobial Stewardship Pharmacist, Pharmacy/Infectious Diseases, Auckland DHB and Dr Rosemary Ikram, Clinical Microbiologist, Christchurch for expert review of this article.


  1. Johnston BC, Goldenberg JZ, Vandvik PO, et al. Probiotics for the prevention of pediatric antibiotic-associated diarrhea. Cochrane Database Syst Rev 2011:CD004827.
  2. Barbut F, Meynard JL. Managing antibiotic associated diarrhoea. BMJ 2002;324:1345–6.
  3. Butler CC, Duncan D, Hood K. Does taking probiotics routinely with antibiotics prevent antibiotic associated diarrhoea? BMJ 2012;344:e682.
  4. Wilton L, Kollarova M, Heeley E, et al. Relative risk of vaginal candidiasis after use of antibiotics compared with antidepressants in women: postmarketing surveillance data in England. Drug Saf 2003;26:589–97.
  5. Videlock EJ, Cremonini F. Meta-analysis: probiotics in antibiotic-associated diarrhoea. Aliment Pharmacol Ther 2012;35:1355–69.
  6. Pirotta M, Gunn J, Chondros P, et al. Effect of lactobacillus in preventing post-antibiotic vulvovaginal candidiasis: a randomised controlled trial. BMJ 2004;329:548.
  7. Snydman DR. The safety of probiotics. Clin Infect Dis 2008;46:S104–11.
  8. Sharma P, Tomar SK, Goswami P, et al. Antibiotic resistance among commercially available probiotics. Food Res Int 2014;57:176–95.