Educational information, not medical advice — ask your doctor or midwife.
Cuasia is the bitter wood of Quassia amara, a small tree of Central and South America. The wood is so bitter that the plant is called bitterwood. In much of Latin America it goes by hombre grande.
It is used mainly as a bitter tonic taken before food. Folk use links it to poor appetite, slow digestion and gut worms. A wash made from it is also put on the scalp for head lice.
Human evidence is thin. A few small studies back a lice lotion; most other uses rest on tradition alone. Long traditional use is not proof that a remedy works, and it is not proof that it is safe.
The bitter taste is the part best understood. Bitter compounds on the tongue may set off saliva and digestive juices. This "bitter tonic" idea is old and plausible, but human proof for cuasia itself is limited.
In animal studies, quassin raised stomach acid output. In test tubes and in animals, extracts slowed some parasites, including malaria parasites. Whether that happens in people at normal amounts is not established.
For head lice, lab tests suggest the extract harms the lice. Small human studies of a quassia lotion reported fewer lice after use. That is the strongest human evidence here, and it is still slim.
Animal work has also shown lower sperm counts in male rats given quassin. This has not been tested properly in people, but it is a real reason for care.
Cuasia is sold as wood chips, shavings, powder, capsules, tinctures and tea bags. Digestive bitters and tonic blends often list it as one ingredient.
A cold water soak of the chips was the usual home form. Carved "bitter cups" were once filled with water, which picked up the taste.
Lotions and shampoos made from the extract are sold for head lice. Garden sprays also use it as an insect killer.
The wood is used as a bitter flavouring in some drinks. Those food uses involve far smaller amounts than a medicinal tea.
Cuasia has a long record in Latin American folk medicine. Healers in Suriname used it for fever, and the plant reached Europe in the 1700s. It was listed in several European pharmacopoeias as a bitter tonic.
In Costa Rica, Panama and Colombia the wood was steeped in water for poor appetite and stomach complaints. It was also a common home remedy for worms.
Farmers used the same bitter extract against insects on crops. In parts of the region it was rubbed on the scalp for lice. None of this history is an instruction to use it yourself.
Stop and see a doctor if you vomit repeatedly, have severe stomach pain, or notice black or bloody stools.
Get urgent care for a fever with chills after travel to a malaria area. Do not treat that at home.
See a doctor for a rash, swelling of the face or mouth, or trouble breathing after any product containing it.
In the United States, cuasia is sold as a dietary supplement and is allowed as a flavouring in some drinks. In the European Union it is used as a bitter flavouring under set limits. Germany's herbal expert panel has listed bitterwood for poor appetite and indigestion. It is not a licensed medicine in the UK.
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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