Educational information, not medical advice — ask your doctor or midwife.
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What is costmary?
Costmary is a tall, mint-scented herb from the daisy family. Its botanical name is Tanacetum balsamita. It is a close relative of tansy.
The long, grey-green leaves smell of balsam and spearmint. Cooks once used them to flavour ale, soups and cakes. German names for it are Frauenminze and Marienblatt.
Here is the honest verdict on evidence. There are no good human trials of costmary for any health problem. Long tradition is not proof that it works, and it is not proof that it is safe.
Treat costmary as a kitchen and garden herb. It is not a treatment for any illness.
How is costmary thought to work?
No one has shown what costmary does in the human body. Everything below comes from lab work, not from people.
The leaf and its oil are rich in scented plant compounds. In test-tube studies, extracts slow the growth of some bacteria and fungi. Whether that happens on skin or in the gut is not known.
In laboratory tests the oil also shows antioxidant activity. That means it mops up reactive molecules in a dish. It tells you nothing about disease in a person.
The old digestive use is usually explained by bitterness and scent. Bitter, aromatic herbs can prompt saliva and gut movement. For costmary itself this has not been measured in people.
Some costmary plants are rich in thujone, a compound also found in wormwood. In animals, thujone can act on the brain and trigger seizures at high doses. That is a reason for caution, not a benefit.
How is costmary sold and used?
Costmary is a garden herb first and a product second. You will not find standardised capsules in most shops.
Common forms:
- Fresh leaves picked from a garden plant - Dried leaf, loose or inside herbal tea blends - A single dried leaf used as a scented bookmark, the old "Bible leaf" custom - Essential oil, sold for fragrance - Tincture (an alcohol extract), from specialist herb suppliers only
Never swallow the essential oil. Any oil is far more concentrated than a leaf, and thujone-rich oil is the real hazard here.
Fresh leaf in cooking is used in very small amounts. The flavour is strong and quickly becomes bitter.
What did tradition use costmary for?
Costmary spread through European monastery and cottage gardens from the Middle Ages onward. Monks and nuns grew it for scent, for flavouring ale and for salves.
The English name links the plant to the Virgin Mary. So do the German names Frauenminze and Marienblatt. Settlers later carried it to North America, where it became the "Bible leaf" pressed into psalm books.
Folk writers listed it for stomach upset, period pain, headache and wounds. Those were the claims of their day. They were never tested the way a medicine is tested now.
By the twentieth century the plant had largely dropped out of use. It survives mostly in heritage gardens and old recipe books.
When to see a doctor
Stop using costmary and call 999 or go to A&E if you have:
- Swelling of the lips, tongue or throat - Trouble breathing or a tight chest - Twitching, fits or confusion after swallowing any oil - Yellow skin or eyes, or dark urine
Stop and book a routine appointment if you get a spreading rash, ongoing nausea, headache or dizziness.
See a doctor rather than self-treating for stomach pain that keeps returning, period pain that stops you working, or any wound that looks infected.
Costmary is not approved as a medicine in the UK, the EU or the US. It is treated as a garden plant and food flavouring herb. EU law caps how much thujone may reach food and drink from flavourings.
Evidence grades and traditional-use labels are explained in our methodology. Nothing here is medical advice.
Reviewed by the HealisticMD editorial team How we grade evidence →
The published evidence for Costmary is weak or absent. That usually means no controlled trial has tested it, not that a trial found it ineffective — the two are different, and we grade only what has been published.
What it is typically used for, usual dose, cautions, and the study behind it where real trial data exists.
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