How much probiotic do you need per day?

The dose at which probiotics show an effect varies considerably depending on the application. In children receiving antibiotics, a dose of 10 billion CFU (colony-forming units) per day had no significant effect on strictly defined antibiotic-associated diarrhoea. That same dose did reduce the overall risk of diarrhoea during and shortly after the course of antibiotics, by roughly 35% relative risk reduction. A fixed 'good dose' therefore does not exist: it depends on what you want to achieve and which strains you are taking.

For recurring gut-bacterial infections (Clostridioides difficile, a dangerous hospital-acquired bacterium), dose clearly mattered. A dose of 8 billion CFU per day reduced relapse from 45.5% to 13.8%. A lower dose of 1.6 billion CFU showed no demonstrable effect. This concerns a specific, experimental probiotic mixture of eight bacterial strains; larger studies still need to confirm this.

In young children with an upper respiratory tract infection, a mixture of three specific bacterial strains taken for two weeks shortened the duration of fever by an average of two days. The available data do not give a fully conclusive answer on the exact CFU dose in this case. In adults with severe depression, 10 billion CFU of one specific Bifidobacterium strain showed a statistically significant effect on mood scores after four weeks, but the study included only 45 participants and is therefore purely exploratory.

Important to know: the WHO explicitly advises against probiotics in children with acute watery diarrhoea, because the evidence for this is too weak and too uncertain. Probiotics are therefore certainly not always the appropriate choice, even for 'obvious' complaints such as diarrhoea in children.