Probiotics have become one of the most popular supplement categories, and there’s good reason for the interest. Research linking gut bacteria to everything from digestion to mental health has exploded over the past decade. But separating genuine benefits from marketing hype takes some work.

I’ve gone through the clinical evidence for 32 claimed benefits. Some hold up well. Others are more wishful thinking than science. Here’s an honest assessment of what probiotics can and cannot do.

What are probiotics?

Probiotics are live microorganisms that, when consumed in adequate amounts, provide health benefits. The most common types belong to the Lactobacillus and Bifidobacterium families, though yeasts like Saccharomyces boulardii also qualify.

Your gut contains roughly 100 trillion microorganisms. That’s not a typo. These bacteria collectively encode over 3 million genes (compared to about 23,000 in the human genome) and produce thousands of metabolites. Around 80% of your immune cells reside in the gut, which partly explains why gut health affects so many seemingly unrelated conditions [1].

The gut microbiota help with nutrient metabolism, vitamin B12 synthesis, preventing pathogen invasion, and maintaining the intestinal barrier. When this community gets disrupted, problems follow. Altered gut bacteria composition appears in cardiovascular disease, cancer, type 2 diabetes, obesity, colitis, asthma, and various immune disorders.

What’s the difference between probiotics and lactic acid bacteria?

Probiotics is the umbrella term for all beneficial bacteria. Lactic acid bacteria (LAB) are one category that produces lactic acid when metabolising sugars. Since about 90% of commercial probiotics are LAB, the terms often get used interchangeably.

Common strains include Lactobacillus acidophilus (the “A” bacteria), Lactobacillus paracasei 33 (LP33), Lactobacillus rhamnosus GG, and various Bifidobacterium species. Different strains have different effects, which is why research on one strain doesn’t necessarily apply to another.

Benefits with reasonable evidence

1. Vaginal infections

Vaginal infections affect an estimated 5% to 70% of women at some point, with bacterial vaginosis and vulvovaginal candidiasis being most common. Left untreated, bacterial vaginosis increases the risk of second-trimester miscarriage, preterm birth, and post-caesarean endometritis.

A meta-analysis of 30 studies found that combining probiotics with standard drug treatment reduced recurrence rates and improved cure rates compared to drugs alone [2]. The combination approach seems more effective than either treatment individually.

2. Acute upper respiratory infections

The common cold is, well, common. A Cochrane review of 13 trials (3,720 participants) found oral probiotics reduced the number of acute upper respiratory infections, shortened illness duration, decreased antibiotic use, and reduced cold-related absenteeism [3].

The evidence quality is moderate, but the direction is consistently positive. Probiotics won’t make you invincible, but they might mean fewer sick days.

3. Allergic rhinitis

Allergic rhinitis affects 10% to 30% of adults, and existing treatments only fully control symptoms in about 39% of patients. A systematic review of 22 double-blind studies (2,242 participants) found probiotics helped alleviate seasonal symptoms including nasal congestion, itching, and runny nose [4].

The LP-33 strain showed particularly promising results. The mechanism appears to involve immune modulation, specifically reducing the Th1:Th2 ratio.

4. Chronic periodontitis

Periodontal disease ranges from treatable gingivitis to severe periodontitis with tooth loss. A meta-analysis of 7 randomised trials found that adding probiotics to traditional scaling and root planning improved periodontal damage indicators, including probing depth and clinical attachment level [5].

Probiotics appear to produce antibacterial compounds and improve the epithelial barrier. The evidence isn’t overwhelming, but there’s enough to consider probiotics as an adjunct to dental treatment.

5. Blood pressure regulation

A meta-analysis of 9 trials (543 participants) found daily probiotics reduced systolic blood pressure by 3.3 mmHg and diastolic by 1.4 mmHg [6]. The effect was strongest with use over 8 weeks, multiple strains, baseline blood pressure above 130/85 mmHg, and daily doses exceeding 10 billion CFU.

Three millimetres of mercury isn’t dramatic, but it’s statistically significant and might be clinically meaningful for people already near hypertension thresholds.

6. Reducing allergies and atopy in children

A meta-analysis of 17 trials (2,947 participants) found continuous probiotic use before and after birth (during pregnancy, breastfeeding, and the first year of life) reduced the risk of atopic reactions and food allergies in young children [7].

This is one of the more consistent findings in probiotic research. If you’re pregnant or breastfeeding with a family history of allergies, discussing probiotics with your doctor seems reasonable.

7. Postoperative wound infections

Wound infections account for 38% of hospital-acquired infections and can increase mortality risk 2 to 11 times. A meta-analysis of 28 studies (2,511 participants) found probiotics during gastrointestinal surgery reduced respiratory, urinary tract, and wound infections [8].

Hospitalisation time and antibiotic duration were also shorter in probiotic groups. A separate analysis of liver transplant patients found similar benefits.

8. Irritable bowel syndrome

Irritable bowel syndrome affects about 11% of the population, causing abdominal pain, bloating, and altered bowel habits. A meta-analysis of 21 studies found probiotics improved overall symptoms and quality of life [9].

Interestingly, small doses of single strains for short periods showed significant effects. More isn’t always better.

9. Blood sugar and diabetes

A meta-analysis of 12 studies (770 participants) found probiotics helped regulate fasting blood glucose and insulin while improving lipid profiles [10]. Multiple strains used for over 8 weeks showed the clearest benefits.

This doesn’t mean probiotics replace diabetes medication. But for people with prediabetes or mild type 2 diabetes, they might be worth considering alongside diet and exercise.

10. Helicobacter pylori eradication

H. pylori causes chronic gastritis, peptic ulcers, and increases gastric cancer risk. Standard triple therapy fails in 10% to 45% of cases due to antibiotic resistance.

A meta-analysis of 30 trials found adding probiotics to triple therapy increased eradication rates by 13.8% and reduced side effects including nausea, vomiting, and diarrhoea [11]. This is one of the better-supported uses.

11. Functional constipation

Functional constipation (the non-disease kind) affects over 14% of adults. A systematic review of 14 studies (1,182 participants) found probiotics improved gut transit time, stool frequency, and stool consistency [12]. B. lactis strains appeared most effective.

12. Antibiotic-associated diarrhoea

Antibiotics disrupt gut bacteria, causing diarrhoea in 5% to 49% of patients. A meta-analysis of 36 trials (9,312 participants) found probiotics reduced this risk regardless of antibiotic type or probiotic duration [13].

If you’re prescribed antibiotics, taking probiotics alongside (not at exactly the same time) is well-supported.

13. Acute diarrhoea in children

A meta-analysis of 84 trials (13,443 children) found several strains helpful for acute diarrhoea, including Saccharomyces boulardii, Lactobacillus rhamnosus GG, and Lactobacillus reuteri [14]. S. boulardii appeared most effective overall.

14. COVID-19 symptom improvement

A meta-analysis of 7 clinical trials and 2 cohort studies found probiotics as adjunctive therapy improved symptoms in 51% of COVID-19 patients, particularly cough, headache, and diarrhoea [15]. The sample sizes were small, so treat this as preliminary.

15. Lipid regulation

A meta-analysis of 38 trials found probiotics reduced total cholesterol, triglycerides, and LDL cholesterol, though HDL remained unchanged [16]. The effect sizes were modest but consistent.

16. Postoperative intestinal obstruction

Postoperative ileus delays recovery in 10% to 15% of abdominal surgery patients. A meta-analysis of 21 trials (1,776 gastrointestinal cancer patients) found probiotics shortened time to first flatus, first bowel movement, and first solid food, while reducing abdominal distension [17].

17. Infant gastroesophageal reflux

About 50% to 70% of infants experience reflux at some point. A meta-analysis of 6 trials found Lactobacillus reuteri strains reduced reflux rates [18]. The male-to-female ratio of affected infants is about 2:1, and most cases resolve by 12 months regardless of treatment.

18. Early childhood caries prevention

A meta-analysis of 9 trials (2,363 preschoolers) found probiotics had a statistically significant, if small, preventive effect on early childhood caries [19]. Worth considering for children with high cavity risk.

19. Infantile colic

Colic affects about 20% of infants and drives parents to exhaustion. A meta-analysis of 32 trials (2,242 infants) found Lactobacillus reuteri DSM 17938 reduced crying time [20]. This specific strain has the most evidence.

20. Inflammatory bowel disease

A meta-analysis of 38 trials found probiotics reduced disease activity in ulcerative colitis and contributed to remission [21]. Formulations with lactic acid bacteria, bifidobacteria, or multiple strains worked best, with daily doses exceeding tens of billions of CFU.

Crohn’s disease results were less impressive. Probiotics seem to help ulcerative colitis more than Crohn’s.

21. Chemotherapy-induced diarrhoea

Diarrhoea affects 50% to 80% of chemotherapy patients and can lead to treatment interruptions. A meta-analysis of 23 trials (2,570 patients with abdominal or pelvic cancers) found probiotics reduced severe diarrhoea (grade 2 or higher) [22].

22. Colorectal cancer surgery recovery

A meta-analysis of 17 studies (1,242 participants) found probiotics protected the intestinal mucosa after colorectal cancer surgery [23]. The specific strains and duration need more research.

Benefits with mixed or weak evidence

I’ll be honest about these next findings. The research exists, but it’s either preliminary, inconsistent, or the effect sizes are small.

23. Weight loss

A meta-analysis of 20 trials (1,411 overweight or obese participants) found probiotics reduced BMI by 0.73 kg/m², waist circumference by 0.71 cm, and hip circumference by 0.73 cm. But body weight itself dropped only 0.26 kg, which wasn’t statistically significant [24].

I wouldn’t rely on probiotics for weight loss. The effects are too small to matter practically.

24. Psychological stress

A meta-analysis of 7 trials (1,146 healthy participants) found probiotics reduced subjective stress levels and improved stress-related anxiety [25]. The sample sizes are small, and it’s hard to measure “stress” objectively.

25. ADHD and Asperger’s prevention

One preliminary study found pregnant women taking Lactobacillus GG had children with no ADHD or Asperger’s diagnoses, compared to 17% in the placebo group [26].

One study isn’t conclusive. The finding is interesting enough to warrant more research, but I’d wait for replication before drawing conclusions.

26. Celiac disease symptoms

A meta-analysis of trials involving 5,279 people with coeliac disease found probiotics improved gastrointestinal symptoms on one assessment scale but not others [27]. The evidence is inconsistent.

27. Recurrent aphthous stomatitis (mouth ulcers)

A meta-analysis of 7 trials found probiotics alone relieved oral pain but not ulcer severity. Combining probiotics with steroids or anaesthetic gels was more effective than gels alone [28].

28. Sleep quality

A meta-analysis of 14 trials found probiotics improved subjective sleep quality scores but didn’t affect objective measures like sleep efficiency or latency [29]. You might feel like you sleep better without actually sleeping better. Make of that what you will.

29. Immune function

A meta-analysis of 18 trials found Lactobacillus plantarum increased anti-inflammatory IL-10 while reducing pro-inflammatory cytokines [30]. The clinical significance isn’t clear yet.

30. Oral hygiene

Systematic reviews show probiotics reduce caries-causing bacteria counts and improve some periodontal indicators [31]. But reducing bacterial counts doesn’t guarantee disease control.

31. Alzheimer’s disease

A meta-analysis of 3 small trials (161 elderly patients) found probiotics didn’t improve cognitive function in Alzheimer’s disease [32]. They did improve some metabolic markers, but that’s not why anyone takes them.

32. Sleep quality (paraprobiotic study)

Another analysis found paraprobiotics (dead bacteria) might improve subjective sleep quality, but again, objective measures didn’t change [29]. The gut-brain axis research is fascinating, but practical sleep applications remain unproven.

Side effects of probiotics

For most people, probiotics are quite safe. The NHS notes that common side effects include temporary bloating and gas when starting supplementation [33]. Starting with a low dose and gradually increasing can minimise these effects.

Some yeast-based probiotics (like Saccharomyces) have been associated with constipation and increased thirst. If side effects persist beyond a few weeks, stop taking the supplement and consult a doctor.

Safety precautions

Not everyone should take probiotics freely:

Immunocompromised patients, those with central venous catheters, recent surgery, acute pancreatitis, or critical illness should consult a doctor before using probiotics. Rare cases of systemic infection have occurred in these populations.

Small intestinal bacterial overgrowth (SIBO) patients may experience worsened symptoms. Speak to a gastroenterologist first.

Histamine intolerance sufferers should avoid certain strains that produce histamine in the digestive tract, including Lactobacillus buchneri, L. helveticus, L. hilgardii, and Streptococcus thermophilus.

Fermented food sensitivity: Some probiotic-rich foods like yogurt, sauerkraut, and kimchi contain biogenic amines (histamine, tyramine) that can trigger headaches in sensitive people. Probiotic supplements may be better tolerated.

Allergen content: Some probiotic supplements contain yeast, dairy, eggs, soy, lactose, or prebiotics (lactulose, inulin, oligosaccharides) that may cause reactions in allergic or intolerant individuals. Check labels carefully.

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