Around two million of us swallow what we believe to be "friendly bacteria" in the form of a probiotic supplement every day. But are we wasting our time? One expert this week claimed that up to half of probiotics don't work. "They've got the wrong bacteria or the wrong numbers," said Professor Glenn Gibson of Reading University. "Some have pathogens in them and some are completely sterile, which is quite an achievement for any food product." Gibson says consumers should stick to products which contain 10 million so called "friendly bacteria" from the lactobacillus or bifidobacteria families per dose. These include top brands by Müller, Actimel and Yakult.

It is the second time in a week that supplements have been called into question. A report in the New Scientist suggested that antioxidant vitamin pills, such as A, C and E, do not possess the disease-fighting properties that their makers claim.

But how can manufacturers get away with such trickery? "In the UK, the supplement industry is regulated by the Food Standards Agency and the Medicine & Herbs Regulatory Authority [MHRA]," says John McKee, chairman of the National Association of Health Stores. "Food supplements come under food labelling laws and cannot make medicinal claims." However, sometimes misleading labelling can slip through the net. Steps are under way to tighten the legislation, with the EU Food Supplements Directive and the Traditional Herbal Medicine Directive to be introduced before the end of the decade.

So what should you look for on the label?

Fish oils

What they are
Extracted from oily fish such as mackerel, they are the richest source of omega-3 fatty acids, which cannot be made by the body and must be obtained through the diet.

What they claim to do
Improve concentration, coordination and behaviour in children, ease arthritic joints and prevent blood-clotting.

What to look for on the label
The key omega-3s are DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid). Choose a supplement with at least 18% EPA and 12% DHA.

Does it work?
Experts recommend that 6% of our calorie intake should come from fatty acids, with around 0.5g per day coming from omega-3 fats. Omega-3 deficiency, coupled with high intakes of saturated fats, has been associated with heart disease. Fish oils are certainly helpful to joints but the jury is still out on whether they boost mental functioning in children.

Glucosamine sulphate

What it is
A building block for protein, needed to manufacture glycosaminoglycan molecules, found in cartilage.

What it claims to do
Ease the pain of arthritic or injured joints.

What to look for on the label
According to Dr Kate McLintock, an advisor to the Arthritis Research Campaign, a therapeutic dose is between 1200 and 1500mg per day. "Make sure that is at least 1000mg of pure glucosamine," McKee says. "Sometimes the figure includes potassium chloride, which is added to stabilise the product."

Does it work?
The latest and largest study into its effects, the GAIT (glucosamine arthritis intervention trial), published this year, found glucosamine proved no better than a placebo in relieving osteoarthritic knee pain. However, the research did show a smaller subgroup of patients with moderate to severe osteoarthritic knee pain who experienced significant pain relief when they took chondroitin sulphate as well.

Garlic

What it is
Garlic is composed of sulphur-containing substances, one of which is allinin. When garlic is crushed, the allinin becomes allicin.

What it claims to do
It is thought that allicin may affect the way the body deals with cholesterol, lowers blood pressure and reduces the blood's tendency to clot. Garlic also contains immune-boosting antioxidants.

What to look for on the label
A dose of 300mg of dried garlic providing 1350μg of the active ingredient allicin per day has proven effective in most positive studies.

Does it work?
According to the British Heart Foundation, there has not been clear evidence to show that garlic reduces the risk of developing or dying from coronary heart disease.

St John's wort

What it is
A common hedgerow plant (Hypericum) producing yellow flowers. It contains at least 10 active chemicals.

What it claims to do
Relieves depression.

What to look for on the label
Check that there is enough of the active ingredient. A daily dose should provide 300mg standardised extract of St John's wort (900μg of the active ingredient hypericin). It is recommended to take St John's wort for 10-14 days for it to have an effect.

Does it work?
There is evidence that it is effective against mild and moderate depression. It seems to affect the levels of brain chemicals, especially serotonin and noradrenaline.

Melatonin

What it is
A hormone produced in the brain's pineal gland that is best known for controlling the wake/sleep cycle. Levels rise in the evening, and subside in the morning.

What it claims to do
Helps counter sleep deprivation and jetlag.

What to look for on the label
Melatonin can't be bought in UK shops,but, it is widely available on the internet or in the US. Look for dosages of 2mg of melatonin which should be taken on the first evening you arrive in a new time zone. Continue this dosage for five days.

Does it work?
Several trials have shown it can help prevent jetlag. This year a team at Washington University found that taking 3mg daily (a higher than normal dose) for four weeks, then coming off the drug for the following four, could be useful in treating tinnitus. Another study showed it eases seasonal affective disorder.

Ginkgo biloba

What it is
Comes from the leaves of the Ginkgo biloba tree, the oldest living species of tree. It is the most commonly used herbal medicine in Europe.

What it claims to do
Prevents age-related cognitive decline, slows the progression of Alzheimer's and improves cognitive function. There are even claims its circulatory powers boost erectile function.

What to look for on the label
Products containing 120-240mg of extract of ginkgo (standardised to contain 6% terpene lactones and 24% flavone glycosides, both active ingredients) per day in two or three portions.

Does it work?
"There are plenty of studies showing it is helpful, but nothing conclusive," Sutton says. Two years ago a study at the University of California reported that a small group of MS patients treated with ginkgo biloba had improved memory and mental function, compared with placebo-treated patients.

Echinacea

What it is
A big purple flower that looks like a daisy and is native to the prairies of the western United States.

What it claims to do
Cure the common cold.

What to look for on the label
The World Health Organisation suggests that 300mg doses appear most effective for consumers.

Does it work?
Evidence is inconclusive, although many swear by it. Last year, a good study reported that echinacea was no more effective than a placebo in curing 437 people with the cold virus.