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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.

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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.

How to take it

TimingTiming does not matter; external use only.
With food?Not applicable — this is an external preparation and should not be swallowed.
Worth knowingTinctures sold in dropper bottles look like oral products but are not; taking usnic acid by mouth is the exposure that has caused liver failure.

Interactions

With medications

  • paracetamol (acetaminophen), methotrexate, isoniazid and other liver-toxic drugs

    Oral usnic acid has caused severe hepatotoxicity on its own, including fatal and transplant-requiring cases. do not take it orally at all, and never alongside these drugs or alcohol

With other supplements

  • kava, comfrey, high-dose green tea extract and other liver-stressing supplements

    Additive hepatotoxic burden with no offsetting benefit. do not combine

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does usnea work for infections?

There is no clinical evidence: usnic acid from usnea kills bacteria in the test tube, but no human trial has tested usnea for respiratory or urinary infections, so the evidence is limited.

Is it safe to take usnea by mouth?

No. Oral usnic acid has caused severe liver injury, including acute liver failure requiring transplantation and fatal cases, so do not take usnea by mouth, even if a tincture comes in a dropper bottle.

Can usnea be used on the skin?

Usnea's traditional use is external, as a tincture or wash applied to skin, which is the only use its lab antibacterial activity relates to. Bacterial respiratory infections still need proper medical assessment.

Scientific references

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