The probiotics with the strongest evidence for relieving constipation are strains of Bifidobacterium and Lactobacillus, with Bifidobacterium lactis HN019 standing out for the most consistent results in adults. But “best” depends on your specific situation, your age, and the type of constipation you’re dealing with. Several well-studied strains have shown real improvements in how often people go and how comfortable the process is.
How Probiotics Help You Go
Probiotics don’t work like a laxative. They produce short-chain fatty acids, especially butyrate, by fermenting fiber in your colon. Butyrate directly stimulates the nerve cells lining your gut, increasing the muscular contractions that move stool along. In germ-free animals (raised without any gut bacteria), gut motility is significantly reduced, and infusing butyrate into the colon partially restores it.
Over the longer term, butyrate increases the number of nerve cells in the gut that produce acetylcholine and substance P, two signaling molecules that drive intestinal contractions. Probiotics also trigger the release of a gut hormone called PYY, which influences how much water your intestines absorb. The net effect is faster transit time, softer stool, and more regular bowel movements.
Bifidobacterium Lactis HN019: Best Overall for Adults
This is the strain with the most direct evidence for functional constipation in adults. In a study of 100 constipated adults, daily supplementation at both a high dose (about 17 billion CFU) and a low dose (about 1.8 billion CFU) for two weeks reduced colonic transit time. After four weeks, the high-dose group gained an average of 2 extra bowel movements per week, and the low-dose group gained 1.7. That’s a meaningful difference when you’re only going a few times a week to begin with.
What makes HN019 particularly appealing is that even the lower dose produced results. Many probiotic strains require very high colony counts to show any benefit, but HN019 appears to work across a wider dosage range. Look for it listed as “Bifidobacterium animalis subsp. lactis HN019” on supplement labels.
Lactobacillus Casei Shirota: Good for Stool Consistency
This strain, best known as the bacteria in Yakult drinks, has a slightly different strength. In a four-week study, daily supplementation increased bowel movements from about 4 to 5 times per week across all participants. But the more interesting finding was its effect on stool consistency.
Participants who started with hard, dry stools saw their consistency scores improve significantly, moving from hard pellets toward a softer, more normal form. Meanwhile, participants who started with loose stools actually saw their consistency firm up slightly. This normalizing effect makes Shirota a reasonable choice if your constipation involves hard, difficult-to-pass stools rather than just infrequent trips to the bathroom.
Lactobacillus Reuteri DSM 17938: Best for Infants
For babies and young children with chronic constipation, this is the most studied strain. In a double-blind trial of 44 constipated infants, those receiving L. reuteri DSM 17938 had significantly more bowel movements than the placebo group by week 2, with the benefit continuing through week 8. The improvement was specifically in frequency. Stool consistency didn’t change much, but the babies were going more often.
This strain is widely available in infant-specific probiotic drops, making it easy to administer. It’s worth noting that the study found no change in crying episodes, so if your baby is both constipated and colicky, the probiotic may only address one of those problems.
Combining Probiotics With Prebiotics
Products that pair a probiotic strain with a prebiotic fiber like inulin or fructooligosaccharides are called synbiotics, and the logic behind them is sound: the prebiotic feeds the probiotic, potentially amplifying its effects. Clinical trials do show that synbiotics can reduce transit time, increase bowel movement frequency, and improve stool consistency.
The catch is that the evidence is still thin compared to probiotics alone. Only about 10 human studies have evaluated synbiotics specifically for constipation, and results on related symptoms like bloating and abdominal discomfort are mixed. Some people find the added prebiotic fiber helpful, while others find it increases gas. If you already eat a fiber-rich diet, a standalone probiotic may be all you need. If your diet is low in fiber, a synbiotic product could offer additional benefit.
Dosage and How Long to Wait
For most constipation-related strains, studies show benefits at doses of 1 billion CFU and above, though research across gastrointestinal conditions generally finds that 5 billion CFU or higher tends to be more effective. The HN019 studies used doses between 1.8 billion and 17 billion CFU, and both worked, so more isn’t always dramatically better.
Timing matters more than most people expect. Some people notice changes within a few days, but clinical trials typically measure outcomes at 2 to 4 weeks. The HN019 study saw transit time improvements at 2 weeks and the full bowel frequency increase at 4 weeks. Give any probiotic at least a month of consistent daily use before deciding it isn’t working for you. Skipping days or switching products every week doesn’t give any single strain a fair shot.
Side Effects to Watch For
Most people tolerate constipation-targeted probiotics well, but some experience increased gas and bloating during the first week or two as their gut microbiome adjusts. This is usually mild and temporary.
In rare cases, the effects can be more significant. Researchers at Augusta University found that in a group of 38 patients reporting gas and bloating from probiotic use, 30 also experienced brain fogginess, including confusion and difficulty concentrating that lasted anywhere from 30 minutes to several hours after eating. The cause was bacterial overgrowth in the small intestine, where probiotic bacteria had accumulated and were producing excess gas, including compounds that can affect brain function. Some patients described symptoms severe enough to interfere with work.
This doesn’t mean probiotics are dangerous for most people, but if you develop worsening bloating, significant abdominal distension right after meals, or mental fogginess after starting a probiotic, stop taking it and talk to your doctor. These symptoms resolved in the study participants once they discontinued their probiotics.
Choosing the Right Product
The strain designation matters more than the brand. A product listing only “Lactobacillus acidophilus” without a specific strain code (like the “HN019” or “DSM 17938” in the strains above) makes it impossible to know whether it matches what was actually tested in clinical trials. Different strains of the same species can have completely different effects.
Look for products that list the full strain name, guarantee a specific CFU count through the expiration date (not just “at time of manufacture”), and store appropriately. Some strains require refrigeration while others are shelf-stable. A product that sat in a hot warehouse for weeks may contain far fewer live bacteria than the label claims.
For most adults with occasional or functional constipation, starting with Bifidobacterium lactis HN019 at 1 billion CFU or above, taken daily for at least four weeks, is the most evidence-backed approach. If hard stools are your primary issue, Lactobacillus casei Shirota is a solid alternative. For infants, Lactobacillus reuteri DSM 17938 has the clearest support.