Probiotics: which ones work, taking them with antibiotics and how to read the label

What probiotics are and why the strain matters

Probiotics are live microorganisms that benefit health when taken in adequate amounts. That definition tells you nothing about the bottle in your hand: benefits are shown for one strain in one situation.

Every probiotic has three levels of name:

• genus – for example Lacticaseibacillus or Bifidobacterium;
• species – rhamnosus, lactis, longum;
• strain – a letter-and-number code such as GG or DN-173 010.

Two strains of the same species can behave differently. So “lactobacilli” on a label without a strain code tells you very little. You can’t match that product to any trial.

Amounts are given in CFU (colony-forming units), the number of live cells. Typical supplements provide 1–10 billion CFU per serving, and some reach 50 billion. According to NIH ODS, a higher count doesn’t mean a bigger effect.

Which probiotics work: what’s been studied and what hasn’t

The best probiotic is the one that’s been tested for your problem. The table covers the reasons people most often pick up a bottle at the chemist.

Pick your situation to see what’s been studied and when to see a doctor first.

Probiotics after antibiotics: what the evidence says

Up to 30% of people on antibiotics get diarrhoea, and it’s the best-studied use for probiotics.

Children. A 2019 Cochrane review pooled 33 trials with 6,352 children. Diarrhoea occurred in 8% of children given a probiotic and 19% of those without. About nine children needed a probiotic to prevent one case. The benefit showed up at doses of 5 billion CFU a day or more. The authors named L. rhamnosus and S. boulardii as the most appropriate choices.

Adults. In the reviews cited by NIH ODS, S. boulardii cut the rate of antibiotic-associated diarrhoea from 17.4% to 8.2%. L. rhamnosus GG helped significantly only in children. Starting within 2 days of the first antibiotic dose worked better, and no benefit was found in adults over 65.

If your GP has just prescribed antibiotics:

• Start the probiotic alongside the antibiotic, not after the course.
• Pick a strain that has been studied, not a “15-strain blend”.
• Space a bacterial probiotic away from the antibiotic dose, as the label advises.
• Severe or bloody diarrhoea, fever or stomach pain while on antibiotics means you should contact your GP or NHS 111 straight away.

Taking a probiotic to “restore gut flora” after the course has finished, on the other hand, has little solid evidence behind it.

Probiotics for gut health: IBS and bloating

In irritable bowel syndrome (IBS), probiotics on average slightly reduce overall symptoms and tummy pain. A review of 15 trials (1,793 patients) measured the effect after 8–10 weeks. Multi-strain products showed benefit more often than single strains.

In its 2021 guideline, the British Society of Gastroenterology says people with IBS can try a probiotic for up to 12 weeks. If symptoms don’t improve, stop. It doesn’t name a strain because the trials differ too much.

New bowel symptoms after 50, blood in your stool, unexplained weight loss or anaemia need a GP appointment first, not a probiotic.

Kefir, yoghurt and sauerkraut instead of capsules?

Kefir, live yoghurt and sauerkraut contain live cultures and are a normal part of a varied diet. But the pack rarely names a strain or gives a count, and pasteurisation or long storage can leave few live cells. So results from capsule trials don’t carry over to a supermarket yoghurt.

Kefir and yoghurt without much added sugar are a good everyday choice. To head off diarrhoea during antibiotics, though, a studied strain at a known dose makes more sense.

The best probiotic for women: cystitis and “women’s health”

Shelves are full of probiotics marketed to women. But in a 2015 Cochrane review (9 trials, 735 people), probiotics didn’t significantly reduce repeat urinary tract infections compared with placebo. The trials were small, so a benefit isn’t ruled out, but it isn’t shown either. If you keep getting cystitis, see your GP.

How to read a probiotic label: genus, species, strain, CFU

Tap the lines on the sample label below. Each one explains what to check before you buy.

A good label gives a strain code for every microorganism and guarantees CFU to the end of shelf life. If it says “at time of manufacture”, there may be noticeably fewer live cells by the time you take it.

Most EU countries treat the word “probiotic” on food packs as an implied health claim, and the EU has authorised none that uses it. The one related claim – live yoghurt cultures help digest lactose – avoids the word. A few countries, including Spain, Italy and the Czech Republic, allow the term. That’s why many European packs say “live cultures” instead.

How to store probiotics

Storage conditions are set by the manufacturer and printed on the bottle:

• Keep refrigerated – put it in the fridge as soon as your parcel arrives.
• Shelf stable – room temperature, but away from heat, sunlight and damp.
• Close the lid tightly and don’t decant capsules into a pill organiser for long.
• Don’t keep the bottle in the bathroom or next to the hob.

Who should avoid probiotics, and when to be careful

Probiotics rarely harm healthy people. Usual side effects are wind and bloating in the first few days. Serious complications have been reported in very ill people.

• Weakened immunity (chemotherapy, transplants, high-dose steroids): bloodstream bacterial and fungal infections have been reported. Only if your specialist agrees.
• Central venous lines, intensive care, tube or intravenous feeding: many of the 60-plus published cases of fungal bloodstream infection linked to yeast probiotics were in these settings.
• Premature babies: in 2023 the US FDA warned of serious infections in premature infants given probiotics, including one death.
• Children: the AGA (US) advises against probiotics for acute gastroenteritis in children. For a child on antibiotics, ask your GP or pharmacist about the product and dose. More in our guide to vitamins for kids.
• Pregnancy and breastfeeding: check with your midwife or GP, especially if there are complications.

A probiotic doesn’t replace treatment. If you keep getting ill, start with what actually affects immunity: enough vitamin D and, if your diet is short of it, zinc.

How to choose a probiotic on iHerb

1. Decide what it’s for: antibiotics, IBS or something else. For “general health” there’s almost no evidence.
2. Find the full strain on the label: genus, species and code. Without the code you can’t compare it with trials.
3. Check CFU per serving and the wording: “at expiration” beats “at time of manufacture”.
4. Check storage. If you’re travelling without a fridge, choose shelf-stable.
5. Compare price per serving, not per bottle: a serving may be 1 or 2 capsules.

More on Supplement Facts panels in our label-reading guide, and on ordering to the UK in our guide to iHerb UK delivery.