Standing by the counter at the pharmacist waiting to pick up my prescription, I couldn’t help noticing the prominent display of probiotics on the counter. It was two years ago, and I was reading everything I could find on microbiomes and probiotics – whether in books, journals or in shops – in preparation for writing my book The Microbiome: What Everyone Needs to Know.

For days I had focused just on probiotics and here they were, temptingly in front of me, ready for me to buy. The packaging was so glossy and it’s claims so intriguing, I found myself picking up the box to see what they were saying.

“Supporting gut health.” “Friendly bacteria.”

I was about to get antibiotics for my tonsillitis. Should I get some probiotics? I’d heard they might help replace the “good” gut bacteria that antibiotics can wipe out.

The pharmacist knew me by sight, partly because he had just looked down my throat and prescribed them for me and partly because I’m a local GP. He nodded encouragingly and pointed at the display. “These are very popular,” he said.

I turned the box over. The packaging did best when describing what it contained. Thirty capsules to be taken every day, each containing 5 billion live cultures. I compared it with the others on the shelf. Some contained 2 billion, some 10 billion. One contained 25 billion bacteria per capsule. It was........

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