Supplement analysis backed by scientific evidence · PT

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Limited evidenceImmunity

Usnea (Old Man's Beard)

A lichen whose usnic acid kills bacteria in a test tube. There are no human trials for respiratory infection, and usnic acid has a documented record of severe liver injury when taken orally.

Evidence last reviewed: August 2026

What the science says

  • Antibacterial activity in vitro Limited evidence

    A review of usnic acid and its derivatives describes consistent activity against Gram-positive bacteria, including resistant strains, in laboratory testing, which is why the lichen has traditional use as a wound and infection remedy. None of this work involves oral dosing in people.

  • No clinical evidence for infections Limited evidence

    No randomized or controlled human trial has tested usnea or usnic acid for respiratory or urinary infections, so the immune and infection claims are extrapolated from cell-culture data.

  • Documented liver toxicity Moderate evidence

    Reviews of usnic acid hepatotoxicity summarize case reports of acute liver failure, some requiring transplantation, linked to supplements containing it, together with mechanistic work showing mitochondrial uncoupling in liver cells. This is the best-documented human effect of the compound.

Dosage & safety

Studied doseNo safe or effective oral dose has been established. Traditional external use is as a tincture or wash applied to skin. Given the liver risk, oral supplementation is difficult to justify.
SafetyOral usnic acid has caused severe hepatotoxicity, including fatal and transplant-requiring cases, most notoriously in weight-loss products. Do not take it orally, and avoid it entirely with alcohol, paracetamol, other hepatotoxic drugs, existing liver disease, in pregnancy and in children. Lichens also concentrate heavy metals from air and substrate. Bacterial respiratory infections need proper medical assessment.

Scientific references

Where to buy

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