Mullein Leaf (Verbascum thapsus): A Botanical Guide

05 Safety, cautions and interactions

Start here: the data is limited. That is the honest headline for mullein, and it is worth stating plainly rather than glossing over. There is no substantial body of modern clinical safety data for mullein leaf taken as a concentrated oral extract, and the formal European regulatory work covers the flower rather than the leaf. What follows is what has actually been published, not reassurance.

What the regulators say

The European Medicines Agency's Committee on Herbal Medicinal Products has adopted an EU herbal monograph on mullein flower (Verbasci flos), finalised in March 2018. It is a traditional-use monograph, which is a legal category based on long-standing use rather than on clinical trial evidence — the EMA assessment report notes that the clinical data required for well-established use is not available. The monograph's safety positions are these:

• Contraindication: hypersensitivity to the active substance.
• Children and adolescents: use in children under 12 is not recommended, because there is not enough data to establish it.
• Pregnancy and breastfeeding: safety has not been established, and in the absence of sufficient data use is not recommended. No fertility data is available.
• Interactions: none reported. Note that "none reported" is not the same as "none exist" — it reflects an absence of studies as much as an absence of problems.
• Undesirable effects: none known; if adverse reactions occur, a doctor or qualified healthcare practitioner should be consulted.
• Overdose: no case has been reported.
• Preclinical data: adequate tests on reproductive toxicity, genotoxicity and carcinogenicity have not been performed.
• Duration: the monograph frames traditional mullein flower preparations as short-term self-care, directing people to consult a doctor or qualified healthcare practitioner if they are still using the product after about a week.

The iridoid and aucubin question

Verbascum species contain iridoid glycosides, of which aucubin is the best known. In 2021 the EFSA FEEDAP Panel assessed a great mullein tincture proposed as a feed additive. It took the view that, in the absence of experimental genotoxicity data on aucubin, aucubin could plausibly damage DNA, and that other iridoid glycosides likely to be present may behave similarly. On that basis the panel could draw no conclusion on the safety of the tincture for long-living animals, while considering it safe at the proposed level for short-lived fattening animals, where lifetime cancer risk is not the governing concern. In the same opinion the panel found no expected safety concern for the consumer from residues in food-producing animals, and it could reach no conclusion on whether the tincture is a skin or eye irritant or a skin sensitiser, because the data was not there.

That opinion is about animal feed, not about human supplements, and it should not be read across directly. But it is the most recent formal genotoxicity discussion on this plant, and it points the same way as the EMA monograph: the genotoxicity question for mullein is open, not closed. This is a reason to take a supplement like this at the stated dose for defined periods rather than continuously and indefinitely, and a reason not to treat "natural" as a synonym for "tested".

Who should not take it

• Anyone with a known hypersensitivity or allergy to mullein or other Verbascum species, or who has reacted to plants in the figwort family.
• Anyone who is pregnant, planning a pregnancy or breastfeeding, unless a doctor or midwife has said otherwise.
• Children and young people under 18. The EMA monograph does not recommend use under 12 for want of data, and there is no data at all supporting a concentrated 10:1 leaf extract in minors.
• Anyone under active medical care, or being investigated for an undiagnosed problem, without first speaking to their doctor.

Medicines and interactions

No specific medicine interactions have been reported for mullein in the EMA monograph, and the BNF does not carry mullein interaction entries. Again, that reflects how little has been studied. Be particularly careful, and speak to a pharmacist or doctor first, if you take:

• Anticoagulants or antiplatelet medicines.
• Medicines with a narrow therapeutic range, such as lithium, digoxin, ciclosporin, phenytoin or thyroid replacement.
• Diabetes medicines, or anything else where timing around food matters.
• Several medicines at once, or any medicine you have recently started or had a dose change on.

Leave a gap of at least two hours between this supplement and any prescription medicine unless your pharmacist tells you otherwise. Community pharmacists will check a supplement against your full medication list free of charge, and are the right first call.

Duration

There is no established safe upper duration for concentrated mullein leaf extract in humans, because the long-term studies have not been done. A sensible, conservative approach is to use it for a defined period rather than open-endedly, and to review with a healthcare professional if you find yourself taking it continuously for months. Do not exceed one capsule a day. More is not better, and with an unresolved genotoxicity question it is the opposite of prudent.

Before surgery or medical procedures

There is no mullein-specific surgical guidance, but standard practice for supplements applies. Tell your surgeon, anaesthetist and pre-assessment nurse about every supplement you take, including this one, and take the bottle or a photograph of the label with you. Many teams ask patients to stop non-essential supplements one to two weeks before a planned operation. Follow the instruction your own team gives you.

Other practical cautions

• Seeds. Mullein seeds contain rotenone and are not a herbal material for internal use. Our product is made from leaf only. If you gather mullein yourself, do not use the seeds.
• Leaf hairs. The dense hairs on fresh and dried mullein leaves are physically irritating. Contact dermatitis from handling Verbascum thapsus has been reported in the dermatological literature, and unstrained home-made loose-leaf preparations can carry hairs that irritate skin and mucous membranes. This is a reason to prefer a processed extract in a capsule over raw leaf, and a reason to strain thoroughly if you do use loose leaf.
• Wild harvesting. Mullein colonises roadsides and rough ground, where it may be exposed to traffic pollution, spray drift or contaminated soil. Buy from a supplier who tests, rather than picking by a road.
• Stop and seek advice if you develop a rash, swelling, breathing difficulty, digestive upset or any other unexpected reaction. Report suspected side effects from any supplement to your doctor or pharmacist.

None of the above is a complete list, because a complete list would require research that does not yet exist. If you are unsure whether this product is appropriate for you, the answer is to ask a pharmacist before you buy it, not after.