Soapwort is used as an expectorant, a mild diuretic, and to help eliminate toxins from the liver, however, it is toxic and can cause stomach upsets and have a powerful laxative effect when taken in higher doses. It's a very effective herb when applied externally for skin conditions such as eczema, psoriasis, acne, and skin irritations. The herb is also very useful for poison ivy, especially when combined with mugwort.
It has a delightful history - Romans used it as a water softener and during the Middle Ages soapwort (Saponaria officinalis) was known as herba fullonis because fullers (a person who cleaned and thickened freshly woven cloth to remove impurities) used the leaves as a soap to clean cloth). It was probably first mentioned as soapwort in William Turner's book The Names of Herbs in 1548 and today it's also known as bouncing bet and sweet william.
Precious antique fabrics are revitalized with suds made from soapwort leaves and in the Middle East it's still used to clean fragile tapestries. It's also the main ingredient in some homemade shampoos.
While some herbal references suggest it's safe to use internally at the recommended dose others state there have been reports of it being fatal to both human and animals. I have never heard of reports of ill effects, however, it's best not to take the herb internally.
To make a decoction for external use soak 4 tablespoons of dried root (2 tablespoons for fresh) in 1¾ pint/1 litre of cold water for 5 hours then bring to the boil. Simmer for 10 minutes. When cool apply to the affected area as a wash or compress.