Probiotics for Autism: What the Evidence Says

Recently, a parent had written to enquire about the difference between two probiotics (specifically PS128 and lactobacillus reuteri) for the treatment of ASD. I hope to share some of my thoughts on this matter.

The initial idea behind probiotics use in autism is based on the "gut-brain axis" theory  that a constant, two-way line of communication between the digestive system and the brain affects our functioning. We know that many children with ASD also experience frequent and often severe gastrointestinal issues like chronic constipation, diarrhea, and abdominal pain. The theory is that imbalance of bacteria in the gut contributes to inflammation, which affects the brain and worsening ASD symptoms. Theoretically, modifying this balance should help re-balance the gut-brain axis.

The Big Picture: Overall Results Are Disappointing

While the rationale is compelling, the results from high-level research have been disappointing. When researchers pool the data from multiple studies in a systematic review, they are able to balance out positive and negative findings. They find that probiotics, in general, do not cause a significant, reliable change in the core symptoms of ASD (like social communication or repetitive behaviors). This evidence suggests that probiotics are not a broad treatment for the condition itself.

The Small Studies: Signs of Specific, Limited Benefit

Despite the overall negative picture, the two specific probiotic strains discussed, have shown some benefit in very small-scale trials, suggesting they might help with some specific behaviours even if they don't change the overall diagnosis.

1. Lactobacillus plantarum PS128

A randomized controlled trial (RCT) in Taiwan investigated this strain. The study planned for 80 children, which is considered a small pilot study for a complex condition like ASD.

• Key Findings: PS128 did not significantly improve total autism severity. However, the study found limited improvements in opposition/defiance behaviors, particularly in the younger participants. This suggests a potential role in managing challenging behaviors and emotional regulation rather than core social challenges.

2. Lactobacillus reuteri

Other trials have focused on L. reuteri because of its potential link to oxytocin—sometimes called the "social hormone." One pilot study testing a specific combination of two L. reuteri strains involved only 43 autistic children (21 receiving the probiotic), making it an extremely limited dataset.

• Key Findings: Similar to PS128, this intervention did not alter overall autism severity or repetitive behaviors. However, it did show a small but statistically significant improvement in social communication. 

  
  

Conclusion: Limited Evidence but Low Risk Intervention

There is very limited high-quality evidence to support the routine use of probiotics as a primary treatment for the core symptoms of ASD.

However, since probiotics are generally safe and have minimal side effects (usually only minor gas or bloating), a parent may choose to try a specific, well-researched strain for a defined period (e.g., 2–3 months) to see if it helps with a specific, targeted issue like:

• Chronic constipation or diarrhea.

• Oppositional or defiance behaviour (may be linked to PS128).

• Social Communication skills (may be linked to L. reuteri).

If you do not see a clear, measurable benefit after a short trial, the most evidence-based decision would be to stop the treatment.