Grapefruit seed extract

Peel back the hype

The Irish Times
Health Supplement, p. 13
24 March 2009

DOES IT WORK? When grapefruit seed extract is all-natural, it won’t be effective against infections

GRAPEFRUIT SEED extract is made from the seeds, pulp and white membranes that remain after the juice is pressed from grapefruits. The left-over material was originally fed to cattle and pigs, but then it was noticed that these animals seemed to get fewer infections. A method of extracting the plant material left over after the juice was removed was developed.

The extract was at first used to spray on fruit and vegetables to protect them from microbes. Testing revealed that it was safe, and in the 1990s it began to be marketed as a dietary supplement for humans.

The extract is used mostly to treat bacterial, viral and fungal infections. It is especially popular for women to treat the yeast infection, vaginal candidiasis or thrush.

Evidence from studies 

A number of laboratory studies have tested grapefruit seed extracts against dozens of different bacteria, viruses and fungi. Most of these microbes are inhibited or killed by the extract. The doses which are required are low and would be expected to be safe for humans.

However, a search of the medical literature found no studies in which grapefruit seed extract was tested in humans to prevent or treat any type of infection.

Problematic aspects 

A serious problem was revealed in a 1999 study. Researchers at a German Institute of Pharmacy tested six commercially available grapefruit seed extracts. Five of the six products showed strong antimicrobial activity against a range of bacteria, Candida yeast and other microbes.

The researchers examined the extracts further and discovered that the five products which killed the microbes contained a variety of synthetic preservatives. The only extract containing no added preservative was the one which had no antimicrobial activity.

The researchers then made several extracts of their own from fresh grapefruit seeds and juiceless pulp. None of the extracts had any antimicrobial activity.

In the Past 10 years, this same type of analysis has been conducted in Austria, Germany, Sweden, Japan (two separate studies) and the US (three studies). In every study, most or all of the samples contained synthetic preservatives or disinfectants.

At first, manufacturers claimed that the chemicals found in the samples were naturally occurring and appeared similar to the synthetic chemicals. The chances of this being the case are not very likely as studies have now found five different synthetic chemicals in the extracts.

All of these are approved for use in foods and medicines, but in limited amounts. One product was found to contain more that 20 per cent preservative. The antimicrobial activity of grapefruit seed extracts is now widely accepted to be due to these synthetic preservatives and disinfectants.

The effects of consuming grapefruit seed extract containing these preservatives are not known. The analysis of Swedish samples was triggered after two patients had complications related to a blood-thinning medication, warfarin.

Both patients had been taking warfarin for years without problems. Three days after taking grapefruit seed extract, they developed bleeding problems. The medical team had the grapefruit seed extract analysed along with two other commercial products. The three samples contained no trace of grapefruit, but consisted of glycerine, water and benzethonium chloride, one of the preservatives commonly found in these extracts.

Further testing revealed that benzethonium chloride interferes with the metabolism of warfarin, as well as many other drugs.


A remarkable feature of this saga is that problems like those in Sweden have not been reported more often. This may be because grapefruit seed extract is usually diluted significantly before being taken.

It may also be possible that people do not suspect that complications with medicines could be related to supplements they are taking. It is important to let pharmacists, doctors and nurses know about any supplements you might be taking.

When a grapefruit seed extract is all-natural, it won’t treat infections. If it works, it probably contains lots of preservatives. Women who are breast-feeding or pregnant should be particularly cautious as the amount of preservative found in extracts varies considerably.

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