Do Probiotics Actually Work?

1. What does the NHS actually say about probiotics?

Start here, because the NHS is blunter than the tub.

“There’s some evidence that probiotics may be helpful in some cases, such as helping to ease some symptoms of irritable bowel syndrome (IBS).”

Read that again slowly. Some evidence, some cases, some symptoms. The same page says there is little evidence to support many of the health claims made about them.

Then there is a second problem, and it is about the product rather than the biology. The NHS notes that probiotics are generally classed as food rather than medicine. So they skip the testing a medicine goes through. You cannot always be sure a tub holds the bacteria printed on the label, or enough of them to do anything.

That is an odd way to open a guide about probiotics. It is also the honest way.

2. How long before you decide it is not working?

NICE answers this one with a timebox. Four weeks.

“People with IBS who choose to try probiotics should be advised to take the product for at least 4 weeks while monitoring the effect. Probiotics should be taken at the dose recommended by the manufacturer.”

Two details in there are easy to skim past. Take the manufacturer’s dose, not one you invented. And monitor the effect, which only works if you have something to monitor.

The British Society of Gastroenterology landed in a similar place in its 2021 guideline on IBS. Probiotics can be considered. It names no particular species and no particular dose, because the evidence does not support picking one.

So the professional advice amounts to this. Try one properly, for a set period, and watch.

3. Is there a blood number that actually moves?

Most of what probiotics are sold on cannot be measured at home. One thing can.

CRP is short for C-reactive protein. Your liver makes it when something in the body is inflamed, so the number rises and falls with inflammation generally rather than pointing at one organ.

A 2017 analysis pooled 20 randomised controlled trials of probiotic supplements and looked at CRP. Across those trials, CRP came out 1.35 mg/L lower on average, with a 95% confidence interval running from 0.55 to 2.15 mg/L lower.

Now the caveats, which sit in the same paper. The trials disagreed with each other a great deal, measured as an I-squared of 65%. Only 3 of the 20 used the high-sensitivity version of the CRP test. And two other inflammation markers, IL-10 and TNF-alpha, showed no significant change at all.

So the fair summary is a real average effect on one marker, in mixed groups of people, with wide disagreement between studies. That is not a promise about you. It is a reason to measure rather than guess.

If you want to watch your own, the Helvy General Energy & Wellness panel (£149) reports C-Reactive Protein (CRP) alongside vitamin D, vitamin B12, thyroid and cholesterol markers. The Advanced Heart Health panel (£159) reports hs-CRP, the high-sensitivity version, which reads low values more precisely.

4. What if it is not your gut flora at all?

Before you commit three months to capsules, ask a cheaper question. Bloating, cramping and a loose or sluggish gut have several ordinary causes, and two of them have specific tests.

NICE is precise about this. For someone who meets the criteria for IBS, it lists the tests that should be done to rule other things out.

“In people who meet the IBS diagnostic criteria, the following tests should be undertaken to exclude other diagnoses: full blood count (FBC); erythrocyte sedimentation rate (ESR) or plasma viscosity; c-reactive protein (CRP); antibody testing for coeliac disease.”

Only one of those four is something we measure, and that is CRP. The other three belong with your GP: a full blood count, an ESR or plasma viscosity, and the antibody test for coeliac disease. Ask for them before you self-treat, not after a wasted season.

Low iron deserves its own line. It causes a flat, heavy tiredness that people often file under gut trouble, and it is common. Iron, ferritin and folate sit in the Vitamins & Minerals panel (£159).

5. How do you run a fair trial on yourself?

In this order, and the order is the whole trick.

1. Get your baseline before the first capsule, not after.
2. Name the two or three symptoms you care about and score each out of ten.
3. Take one product, at the dose on the box, for four weeks.
4. Change nothing else that month, not the diet and not the training.
5. Score the symptoms again, and retest CRP if it started raised.

If nothing moved, stop. NICE’s own wording is to monitor the effect, and a month of no change is an answer rather than a failure.

One warning about the retest. CRP climbs with any infection, so a cold in week three makes the second reading unreadable. Wait until you have been well for a fortnight.

Frequently asked questions