Educational information, not medical advice — ask your doctor or midwife.
Zi Cao is the dried purple root of gromwell plants. The two plants used are Lithospermum erythrorhizon and Arnebia euchroma. In Chinese medicine it is used mostly on the skin.
It is put on red rashes, slow-healing sores, burns and nappy rash. It is also taken by mouth as a decoction (a strong tea). That use is for rashes said to come from "heat in the blood".
This is a single herb, not a ready-made formula. In traditional practice it is mixed with other herbs. The mix is matched to the person, not taken alone as a supplement.
Here is the honest evidence picture. Zi Cao has a long, well-recorded past in Chinese and Japanese herbal practice. But good human trials are very few.
Traditional use tells you what practitioners have long done. It is not proof that the herb works.
One safety point matters more than the rest. Some Arnebia and Lithospermum roots hold pyrrolizidine alkaloids. These are plant compounds that can harm the liver, so long or high-dose use by mouth is not safe.
No one has shown in people how Zi Cao works. Treat any explanation as a theory, not a fact.
In traditional terms, Zi Cao is called cold and sweet-salty. It is said to "cool the blood" and clear heat and toxins. It is also said to move stuck blood and to loosen the bowels gently.
In lab tests, the purple pigments called shikonins slow the growth of some bacteria (germs). They calm swelling in cell and animal models. They also speed skin repair in animal wounds.
Lab results often do not carry over to people. None of these findings have been confirmed by solid human trials.
The most common form is an infused oil or ointment for the skin. The classic one is Zi Yun Gao, known in Japan as Shiunko. It is a purple salve of Zi Cao and Dang Gui in sesame oil and beeswax.
Other forms you may meet:
Dried root slices or root pieces, simmered with other herbs into a decoction (a strong tea).
Powdered root, stirred into ointment bases or dusted onto weeping skin.
Tinctures and liquid extracts, sold as single-herb products.
Everything it touches turns purple. That includes skin, nails, cloth and bedding. The stain fades but is hard to wash out, so use old sheets and clothes.
Zi Cao appears in the Shen Nong Ben Cao Jing. That is a Chinese herbal text from about the first to second century CE. It has been used ever since for rashes, measles-type eruptions and stubborn sores.
The purple ointment Zi Yun Gao was set down by the Japanese surgeon Hanaoka Seishu around 1800. It is still made today in Kampo (Japanese herbal medicine). That gives the herb a deep record in one linked family of traditions.
See a doctor promptly if your skin turns yellow, your eyes look yellow, your urine goes dark, you itch all over, or you get pain under your right ribs. These can be signs of liver trouble, and you should stop taking the herb straight away.
Also get medical help if you bruise easily, bleed from the nose or gums, or notice bleeding that will not stop.
Get a burn, sore or rash looked at if it spreads, weeps pus, smells bad, comes with a fever, or has not improved after about two weeks. Go to hospital for a deep burn, a burn larger than your palm, a burn on the face, hands or genitals, or any widespread rash with fever.
Evidence grades and traditional-use labels are explained in our methodology. Nothing here is medical advice.
What it is typically used for, usual dose, cautions, and the study behind it where real trial data exists.
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