Educational information, not medical advice — ask your doctor or midwife.
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Astacus fluviatilis is a homeopathic remedy. It is made from the European crayfish, a small freshwater shellfish. In homeopathy it is used mainly for itchy skin rashes, above all hives (raised itchy welts).
Like all homeopathic remedies, it is watered down again and again in a set way. This is called serial dilution. At 30C the source has been thinned so far that no crayfish is likely to be left in the pill at all.
Trials have not shown homeopathic pills to work better than a dummy pill. Long use in one tradition is not proof that a remedy works. Both things are true at once, and this page reports both.
There is no known way this remedy could act on the body once it is diluted to 30C. Nothing of the crayfish is left to act.
Homeopaths work from an old idea called "like cures like". They say a thing that causes a rash in a well person can, when watered down, help a sick person with a like rash. They also say the shaking step at each dilution leaves an imprint in the water.
Chemistry and biology do not support that idea. When people feel better after taking it, the likely reasons are placebo effect (feeling better because you expect to), time, and the fact that hives often clear on their own.
You will most often see:
Small sugar pellets or pills, sold as 6C, 30C or 200C. Liquid drops in a water and alcohol base. Tablets that melt under the tongue.
The mother tincture and very low strengths, such as 1X to 6X, are rare and are the only forms that may still hold crayfish protein. These are the forms that matter if you have a shellfish allergy.
Astacus fluviatilis entered homeopathy in the 1800s in Europe. It is a small, minor remedy, not a famous one.
Old homeopathic texts, such as the materia medica collections of the period, list it for nettle rash that comes with fever, chill or liver complaints. It has stayed a minor entry in that one tradition ever since, and it has no separate record in herbal medicine, Ayurveda or Chinese medicine.
Some reactions to Astacus fluviatilis need urgent care. Call your local emergency number or go to an emergency department straight away — do not wait to see whether it passes.
See a doctor if hives last more than a few days, keep coming back, or spread fast. Also see a doctor for any rash that comes with a fever, for swollen glands that last more than two weeks, or for yellow skin or eyes.
Get emergency help at once if the lips, tongue or throat swell, if breathing gets hard or wheezy, if you feel faint, or if you are sick and dizzy at the same time as a rash. These can be signs of a severe allergic reaction.
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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