Two probiotic strains stand out for constipation relief: Bifidobacterium lactis and Lactobacillus casei Shirota. Both have been tested in multiple clinical trials, and while they work differently, each has shown measurable improvements in how often people go and how comfortable the experience is. Other strains may help too, but these two have the strongest and most consistent evidence behind them.
The Two Best-Studied Strains
Bifidobacterium lactis (sold under sub-strain names like HN019, DN-173010, and Bi-07) has been the most frequently studied probiotic for constipation, appearing in at least seven clinical trials. Its main benefit is increasing how often you have a bowel movement. In one dose-ranging trial, people who were going three or fewer times per week at baseline saw meaningful improvements in frequency. However, higher doses were needed to reduce straining, and its effects on bloating, discomfort, and other gut symptoms are less consistent across studies.
Lactobacillus casei Shirota (often labeled LcS on product packaging) appears to do more than just get things moving. Across five clinical trials, it reduced pain, straining, and the feeling of incomplete emptying. It also improved stool consistency and increased frequency. A review in Frontiers in Nutrition concluded that while B. lactis is strongest for frequency alone, L. casei Shirota improves a broader range of constipation symptoms. If your main complaint is hard, difficult-to-pass stools along with discomfort, L. casei Shirota may be the better fit.
How L. casei Shirota Affects Stool Consistency
One of the more interesting findings about L. casei Shirota is that it doesn’t just soften stool. It normalizes it. In a Chinese clinical trial published in the Journal of Neurogastroenterology and Motility, researchers tracked stool type using the Bristol Stool Form Scale (a 1-to-7 rating where 1 is hard lumps and 7 is watery). After four weeks of daily supplementation, participants with hard stools saw their scores rise from 2.50 to 3.04, moving toward the normal range. But participants who started with overly soft stools saw their scores drop from 4.46 to 4.14. People with already-normal consistency didn’t see significant changes.
This bidirectional effect suggests L. casei Shirota helps regulate colonic transit speed rather than simply pushing everything through faster. For people with constipation specifically, the researchers found it accelerated colonic transit in the hard-stool group.
How Probiotics Speed Up Your Gut
Probiotics don’t work like a laxative. Instead, they produce short-chain fatty acids (particularly butyrate) as they ferment fiber in your colon. These fatty acids trigger a chain of effects that get your intestines moving. Butyrate stimulates the production of serotonin in the gut lining, and about 90% of your body’s serotonin is made in the digestive tract, not the brain. That serotonin activates nerve cells in the intestinal wall, which coordinate the wave-like muscle contractions that push stool forward.
Short-chain fatty acids also support the cells that act as pacemakers for your gut, helping maintain a regular rhythm of contractions. They strengthen the intestinal lining and modulate local immune activity, both of which contribute to healthier motility over time. This is why probiotics tend to produce gradual improvement rather than immediate relief.
What About IBS-Related Constipation?
If your constipation is part of irritable bowel syndrome (IBS-C), the picture is a bit different. Bifidobacterium longum 35624, one of the most studied strains for IBS overall, was tested in an eight-week trial for its effects on various IBS symptoms including constipation. While it helped with other symptoms like bloating and abdominal pain, the improvement in constipation specifically was small and not statistically significant. The researchers noted this may have been because participants didn’t rate their constipation as very severe to begin with, leaving less room for measurable improvement.
For IBS-C, the strains that help with general constipation (B. lactis and L. casei Shirota) are still reasonable options, particularly L. casei Shirota given its broader symptom relief including pain and discomfort reduction. But expectations should be modest. Probiotics for IBS-C tend to work best as one piece of a larger approach that includes dietary changes.
Dosage and How Long to Wait
Most successful constipation trials use doses in the range of 1 billion to 10 billion colony-forming units (CFU) per day, though some have tested higher amounts. In the B. lactis HN019 research, a high-dose group showed benefits that the low-dose group didn’t, particularly for reducing straining. This suggests that for constipation specifically, erring toward the higher end of a product’s recommended dose may be worthwhile.
Clinical trials typically run for four to eight weeks, and meaningful changes in bowel frequency tend to appear within the first two to four weeks. Four weeks is a reasonable trial period before deciding whether a particular strain is working for you. If you see no change after a full month of consistent daily use, switching to a different strain is a logical next step, since the effects are strain-dependent.
Pairing Probiotics With Prebiotics
Prebiotics are the fibers that feed beneficial gut bacteria, and inulin is the most promising one for constipation. It’s found naturally in chicory root, garlic, onions, and bananas, and it’s also sold as a supplement. Combining a prebiotic like inulin with a probiotic strain gives the bacteria a food source that helps them thrive and produce more of those motility-boosting short-chain fatty acids. Products that combine both are sometimes labeled “synbiotics.”
Side Effects in the First Few Weeks
Starting a probiotic can temporarily make things feel worse before they improve. Increased gas, bloating, and even a brief worsening of constipation are common initial reactions. These side effects typically fade within a few weeks as your gut microbiome adjusts.
A meta-analysis of randomized controlled trials found that people taking probiotics had a higher risk of gastrointestinal symptoms compared to those on placebo, with abdominal pain showing the strongest association (roughly 2.5 times the risk). That sounds alarming, but context matters: these studies included people with inflammatory bowel disease, who are more sensitive to gut changes than the average person with functional constipation. For most people, the initial discomfort is mild and temporary.
Starting at a lower dose and increasing over a week or two can help minimize these effects. Taking your probiotic with food, rather than on an empty stomach, also tends to reduce gas and cramping.
Choosing a Product
When shopping for a probiotic for constipation, look for the full strain name on the label, not just the species. “Bifidobacterium lactis” alone isn’t enough. You want to see specific sub-strain designations like HN019, DN-173010, or Bi-07. For L. casei Shirota, the most accessible source is the fermented milk drink Yakult, which contains that exact strain. Other products may list similar species but use strains that haven’t been tested for constipation.
Check that the product guarantees its CFU count “at time of expiration,” not “at time of manufacture.” Probiotics are living organisms, and their numbers drop over time. A product that started with 10 billion CFU could contain far fewer by the time you take it if storage or formulation is poor. Refrigerated products and those with enteric coatings or microencapsulation tend to deliver more viable bacteria to the colon where they’re needed.