The main active ingredient found in greater celandine is the alkaloid chelidonin, a substance not unlike papaverine found in the opium poppy (Papaver somniferum). Chelidonin has an antispasmodic effect and seems to specifically target the bile duct and bronchi. Furthermore, the substance has mild tranquilizing properties.
When greater celandine is used internally, its medicinal effect is rather unpredictable. This could be explained by the great difference in the herb’s quality which is mostly because any herbal preparations quickly lose their medicinal properties when stored and also that there still no proven methods to stabilize the active ingredients.
The herb may retain its properties for up to approximately six months, but if stored longer its effect seems to decline rapidly. This is probably one of the reasons greater celandine has a rather ambiguous status among herbalists.
When the herb is intended for internal uses it is considered to have somewhat limited value and it is seldom used by herbalists on its own but rather in combination with other medicinal herbs.
Today, it is mainly used as an ingredient in herbal teas for its antispasmodic effect on the gallbladder and to stimulate bile flow, but since the content of the alkaloids and other ingredients in the plant varies so much, it is difficult to predict the effect.
Still, there have been several studies that have shown that greater celandine has a rightful place in herbal medicine.
One such study strongly indicated that the herb could be useful in the treatment of cramps-like conditions in the abdomen associated with irritable bowel syndrome.
Another study found that when it was ingested in the form of a cough syrup or extract (equivalent to 15 g of herb pr. day) it resulted in overall improvement in about 80% of a group of patients suffering from chronic bronchitis.
The plant has also been found to be effective treatment for whooping cough.
The German Commission E gives greater celandine the green light and recommends it as a treatment for cramping discomforts related to the bile ducts and the digestive tract.
Greater celandine has been used externally to speed up healing of minor wounds, cuts, and scrapes and as a relief for skin problems such as eczema.
The yellow milk sap from the plant is a strong corrosive substance that can dissolve warts and corns due to the action of protein splitting enzymes.
The herb has also antimicrobial effect and may be helpful as an external treatment for fungal infections.