Around 10 years ago, when I was a pimply, malnourished teenager, my mother returned from the supermarket with a stack of yogurt drinks. They were probiotics, she said, full of “friendly bacteria” that would do me a world of good. Yet when I asked her how exactly they would make me feel better, it soon became clear that she hadn’t a clue. A decade later, we seem no clearer about how the products work and what they do.

Probiotics are said to contribute to a stable balance of good bacteria, known as gut flora, that live in our intestines. This balance helps to make vital nutrients and keeps our digestive systems healthy, but stress, antibiotics, and illnesses that cause diarrhea can upset levels of the flora. Hence the need for probiotics. But does the average human being – when not suffering from such problems – need to increase his quota of friendly bacteria, as the makers of these products would have us believe?

“Topping up your bacteria with fermented milk – of which these drinks are essentially modified versions – has been part of our diet for thousands of years,” said Jeremy Hamilton-Miller, a microbiologist who has studied probiotic supplements. “So it’s perfectly natural to incorporate these yogurts into your diet. But I think the manufacturers need to be a lot clearer about what the supplements actually do.” In other words, general claims about “feeling good” have made consumers think that probiotics will help cure a variety of ailments.

There are, however, specific illnesses that probiotics have been shown to help. Lactobacillus GG, for instance, can help prevent diarrhea, while others can ease constipation. Certain types have been found to relieve the effects of ulcerative colitis, and various strains have been shown to increase levels of vitamin K, to decrease the risk of bowel and colon cancers, and to cut cholesterol levels.

A Swedish study found that babies who were given the probiotic Lactobacillus reuteri during the first six months of their lives were less likely to develop allergies and asthma.

Probiotics also have potential benefits for the elderly, said Jackie Lowden of the British Dietetic Association. “There is evidence that the levels of good bacteria in our bodies decline with age,” she said. “And there have been studies that suggest other benefits; some probiotics are said to suppress quantities of Helicobacter pylori, which has been linked with peptic ulcers and gastric cancers.” Some research shows that probiotics may help with irritable bowel syndrome, she added.

However, many have questioned how many of the good bacteria actually reach the gut, because acid and bile in the stomach are known to kill probiotics. Furthermore, when Mr. Hamilton-Miller carried out a study into the effectiveness of probiotic supplements last year, he found that only a third of the products did what they claimed to do, and one was completely sterile. He will not name the unsatisfactory ones, because he has been threatened with legal action.

“I found that the range of probiotics was extraordinary,” he said. “Some had billions of bacteria in them; one had nothing in it at all; others contained organisms that just shouldn’t have been there.”

But if we do want to take them, is there a way of doing so naturally? The answer is no, although we can ingest prebiotics, the micro-organisms on which probiotics live. These can be found in bananas, asparagus, onions, tomatoes, leeks, cucumber, chickpeas, peas, and sunflower seeds, and some probiotic products now include them.

Mr. Hamilton-Miller said the key to buying probiotics is to buy them long before their sell-by date and to keep them refrigerated. “If you don’t treat bacteria properly, they will die.” He added that probiotics advertised on television or radio are likely to be of high quality, as they are vetted by advertising standards bodies.

“There is obviously a role for probiotics in the diets of certain people,” said Ms. Lowden. “Yet there are huge issues over whether we all need to take them; the fact that they won’t do us any damage means we carry on buying them.”