Are Probiotics Harmless?

For the general population there is convincing evidence that probiotics are very safe; both as dietary supplements and when included in (fermented) foods. Post-marketing surveilance does not indicate any increase in probiotic related infections.

https://www.ncbi.nlm.nih.gov/pubmed/16902895

https://www.ncbi.nlm.nih.gov/pubmed/12410474

https://www.ncbi.nlm.nih.gov/pubmed/16709533

In severely ill patients, however, there is a risk for infection. Probiotics should thus only be used in patients with the consent of the treating physician. In general, the rare probiotic infections that have been reported are easily treated. However, because they are so rare, it may take time before the infective organism is recognised. It is important to realise that this risk/benefit balance is always present when treating patients.

As for other other ‘detrimental’ effects that are sometimes reported, this is often based on epidemiological evidence is causality is thus difficult to prove. Furthermore, the evidence often turns out to be weak, at best.

With the huge range of probiotics available, this is a difficult question! Probiotics include many different strains of bacteria, combinations of strains and different amounts of bacteria. So describing them all as one big group may be doing us a disservice.

In many markets, including the US, probiotic supplements are not regulated very heavily, and therefore do not require significant evidence of efficacy or safety to be sold (https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2702973). This has led to some concern that we may not fully understand the potential risks of these products. Additionally, many studies using probiotics do not adequately report data on adverse events (https://annals.org/aim/article-abstract/2687953/harms-reporting-randomized-controlled-trials-interventions-aimed-modifying-microbiota-systematic).

For most people though, probiotics overall are probably harmless. There is not many published case studies or reviews showing significant risk to healthy people. However, there may be a slightly increased risk to people who are immunocompromised, elderly or very young. For example, one pre-term baby died from a rare fungal infection traced back to a probiotic supplement in 2014.

This is a challenging question as probiotics are defined as a microorganism introduced to the body for its beneficial qualities to the host. So, by definition, a probiotic cannot be harmful. In most cases, probiotics are simply preparations of bacteria that would otherwise be naturally occurring within the body, and as such, it is unlikely that they will pose any danger to the host. However, it is possible that when these bacteria are administered in high amounts, a person can experience side effects related to GI function, including bloating, diarrhea or constipation. These are usually temporary.

In saying that however, there are certain circumstances in which probiotic formulations may have dangerous complications and outcomes if given to vulnerable people. For example, in highly immunocompromised people, there is a small risk that probiotics can promote the translocation of bacteria from the gut into the blood. In people who have compromised immune systems, this can result in systemic blood stream infection – a potentially lethal complication. However, many of the bacteria used in probiotics are what we call anaerobic. This means that they are highly sensitive to oxygen. So often, the types of bacteria used in probiotics would never survive in the blood stream due to the high levels of oxygen.

The way in which probiotics may be dangerous is in their formulation. Whilst the regulations associated with probiotic preparation and use are becoming increasingly strict, they do remain a grey area in terms of their regulatory status. This is also true for the ultimate probiotic, faecal microbiome transplantation (FMT). FMT is the process by which stool is collected from an individual, processed and administered to that same individual or another recipient in order to deliver a large microbial load. FMT has been associated with transmission of certain GI diseases, mainly infection, and other traits in a few select circumstances. For example, FMT has resulted a recipient becoming obese (like the donor), however other factors that may have contributed to this have not been ruled out.

Importantly, these complications have only occurred when the preparation of the FMT has not included appropriate donor and sample screening (to ensure the stool donor does not have existing diseases).