Limited evidenceImmunity

Grapefruit Seed Extract

Marketed as a natural broad-spectrum antimicrobial. Repeated laboratory analyses found that commercial products owe their antimicrobial activity to added synthetic disinfectants, not to the grapefruit.

Evidence last reviewed: August 2026

What the science says

Activity traced to synthetic preservatives Limited evidence
An analysis of commercial grapefruit seed extract preparations found that samples showing antimicrobial activity contained benzethonium chloride, a synthetic disinfectant, while preparations free of such preservatives showed no meaningful activity. This is the central finding in the field and it has not been overturned.
Products repeatedly found adulterated Limited evidence
Liquid chromatography analysis of products labelled as grapefruit seed extract identified benzethonium chloride, methylparaben and triclosan. What is on the label is often not what produces the effect.
Real interaction risk Limited evidence
Case reports describe raised INR and bleeding risk in patients on warfarin who used grapefruit seed extract products. Grapefruit constituents also inhibit CYP3A4, the enzyme that metabolises a long list of common medicines.

Dosage & safety

Studied doseNo meaningful studied dose exists for any health outcome. Labels typically suggest 100–250 mg of extract or 10–15 drops of liquid.

SafetyThe main risks are adulteration with disinfectants such as benzethonium chloride and triclosan, and drug interactions — reported INR disturbance on warfarin, plus the well-known grapefruit effect on CYP3A4 substrates including statins, some calcium channel blockers and immunosuppressants. Undiluted liquid is irritating to mucous membranes. Avoid in pregnancy.

Scientific references

Aspects of the antimicrobial efficacy of grapefruit seed extract and its relation to preservative substances contained
von Woedtke T, Schlüter B, Pflegel P, Lindequist U, Jülich WD — Pharmazie, 1999
Simultaneous identification and quantification by liquid chromatography of benzethonium chloride, methyl paraben and triclosan in commercial products labeled as grapefruit seed extract
Avula B, Dentali S, Khan IA — Pharmazie, 2007
Adverse effects by artificial grapefruit seed extract products in patients on warfarin therapy
Brandin H, Myrberg O, Rundlöf T, Arvidsson AK, Brenning G — Eur J Clin Pharmacol, 2007

Where to buy

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