Galega officinalis is not a true Rue but comes from the Pea Family. In medieval times the herb was mainly used to get rid of internal worms and improve digestion. Traditionally used to treat diabetes the plant has in the 20th century come under closer scrutiny. A quite recent discovery of the properties of this herb found its ability to reduce blood sugar levels, possibly useful in treating diabetes. Goat’s Rue was at the foundation of the biguanide class of antidiabetic drugs, galegine a phytochemical in Galega decreases blood sugar levels, but caused some adverse effects in some studies, this led to the development of other oral antidiabetic drugs related to guanidine, ultimately leading to Metformin, trade name Glucophage, one of the most readily used treatments for diabetes. Be aware Goat’s Rue may interfere with prescribed diabetes drugs, iron absorption and anti-coagulants. I would suggest drinking sparingly and limit yourself to no more than two cups per day. No harmful effects have been reported in humans, but fatal poisoning has occurred in grazing animals that ate large quantities of Goat’s Rue. Tastes nice.
It goes without saying that if you're pregnant (or are under the doctor for anything) caution should be exercised with herbal decoctions.