A probiotic made stools a little softer in a constipation trial. It did not get people to the toilet more often than a dummy powder did

Constipation is one of the most common gut complaints there is. A review of studies using the standard Rome criteria put it at roughly one adult in ten worldwide, and it gets more common with age. Probiotics are an obvious thing to try: a sachet a day, sold over the counter, with the promise of nudging the gut's bacteria in a better direction. The trouble is that each strain of bacteria is its own product, and most have been tested in very few people.

A trial published in Food Science & Nutrition on 1 October 2026 tested one of them. The strain is called Pediococcus acidilactici PA53. Pediococcus is a lactic acid bacterium of the kind found in fermented foods, and it has been studied far more in mice and farm animals than in people.

The trial ran at the General Hospital of Ningxia Medical University in Yinchuan, China, from August 2024 to May 2025. It took adults aged 18 to 70 with functional constipation, meaning no disease was found to explain it: fewer than three bowel movements a week, hard stools, or both. Anyone who had used laxatives, probiotics or antibiotics in the previous four weeks was left out. Eighty-seven people were randomly split into two groups. Each took one 3 gram sachet a day for eight weeks. In one group the sachet held 30 billion PA53 bacteria mixed into dextrin, a starch-based powder. In the other it held the dextrin alone. Neither the volunteers nor the researchers knew who had which until the end. Eighty-one people finished, and four in five of them were women.

The main measure was the Bristol Stool Form Scale, the seven-point chart doctors use to describe stools, from type 1 (separate hard lumps) to type 7 (entirely liquid). Types 1 and 2 count as hard. Both groups started at an average of about 2. After eight weeks the placebo group was still at 2.03. The probiotic group was at 2.76. That gap was statistically clear, and it is the result the paper leads with.

Put it in plain terms, though. A shift of about three quarters of a point means that, on average, stools went from lumpy towards the edge of the normal range, not into it. The authors call the change modest and argue that moving in the right direction matters to people with constipation. That is fair, but it is worth knowing how big the effect was before reading the word 'effective'.

Frequency is the other half of constipation, and here the picture is weaker. At the start, 7 of the 42 probiotic takers who finished had four to six bowel movements a week. By week eight, 16 did. That rise was significant within the group. But the placebo group also rose, from 5 of 39 to 9. Compared directly, the two groups were not clearly different. The trial also asked about quality of life with constipation and about mood, stress and anxiety. Neither changed in a meaningful way in either group, and the two groups ended up in much the same place.

The paper measured a lot more: gut hormones that speed up or slow down the bowel, three markers of inflammation in the blood, and the bacteria in stool samples. Several numbers moved within the probiotic group, and the abstract describes 'beneficial' changes in inflammatory markers. Read the results section and the authors say it plainly themselves: no between-group difference was significant for any of these blood markers. A within-group change only tells you that something shifted over eight weeks, and in this trial the placebo group's hormones drifted the same way. The stool tests did confirm that the bacterium turned up in the gut of people who took it. The variety of gut bacteria in that group actually fell a little over the eight weeks, which the paper reports without making much of it.

There are other limits the authors list. The trial was small, short and at one hospital. Diet was not controlled. The placebo was dextrin, which is a soluble fibre and may itself help the bowel slightly, so the true difference could be a bit larger than measured. Only those who finished were analysed. The probiotic and the placebo were supplied by the company that makes the strain, Wecare Probiotics; the authors report no other funding and no conflicts of interest.

What does this mean if you are constipated? One strain, at one dose, made stools somewhat softer than a dummy powder did over two months. It did not make people go more often or feel better than placebo. The usual first steps (more fibre from food, enough fluid, moving more) have far more evidence behind them. If constipation is new, getting worse, or comes with blood, weight loss or pain, see a doctor rather than a supplement shelf.