Goat’s rue (Galega officinalis) is a flowering legume plant native to the Eastern Mediterranean and Black Sea regions, best known for two things: it inspired the development of metformin, the world’s most widely prescribed diabetes drug, and it remains a popular herbal supplement marketed to increase breast milk supply. The plant has a long history of medicinal use in Europe, though it also carries a reputation as a toxic weed capable of poisoning livestock.
The Plant Itself
Goat’s rue belongs to the legume family, the same plant group as beans, lentils, and clover. It originated in the Eastern Mediterranean and spread through human activity across Europe, Western Asia, North Africa, and eventually to the Americas and New Zealand. The plant produces clusters of small white or purple flowers and has the ability to fix nitrogen in the soil through a symbiotic relationship with bacteria in its roots, which gives it an aggressive growth habit.
That aggressive growth is exactly why many countries classify goat’s rue as a noxious weed. It can take over pastures and roadsides, crowding out native plants. In the United States, it’s considered invasive in several states. While it has ornamental appeal and centuries of medicinal tradition behind it, farmers and land managers generally view it as a problem plant.
The Connection to Metformin
The most significant legacy of goat’s rue has nothing to do with herbal supplements. Chemical analyses of the plant dating back to the mid-1800s found it was rich in a compound called guanidine, particularly in its immature seed pods. In 1918, researchers showed that guanidine could lower blood sugar in animals. That finding kicked off decades of pharmaceutical development.
During the 1920s and 1930s, scientists synthesized several guanidine-based drugs and tested them for diabetes treatment. Most were abandoned because of toxicity, especially as insulin became more widely available. But one derivative, metformin, eventually proved both effective and safe enough for long-term use. Today, metformin is the first-line medication for type 2 diabetes worldwide, and it traces its chemical lineage directly back to the compounds in goat’s rue.
The plant also contains galegine, sometimes called isoamylene guanidine, which was among the early guanidine derivatives studied for blood sugar control. Galegine itself was never developed into a drug, but it helped researchers understand how guanidine-type molecules interact with glucose metabolism.
Use as a Breast Milk Supplement
Goat’s rue is one of the most common ingredients in herbal lactation supplements sold online and in stores. It frequently appears alongside fenugreek and milk thistle in products marketed to breastfeeding parents who want to increase their milk supply.
The evidence behind this use is thin. A few older studies found some positive effects, but none of them meet modern scientific standards. One uncontrolled observational study gave a goat’s rue extract to 336 women with low milk production and reported increases of 30 to 60%. Another unblinded study compared 50 women taking the extract to 50 untreated women, finding that milk output increased by 125% in the treated group versus 75% in the untreated group. But the milk volumes varied widely between individuals, making it hard to draw firm conclusions.
A more recent study tested a commercial product combining goat’s rue with silymarin (a milk thistle extract) and found that treated women averaged 200 mL of daily milk production compared to 115 mL in controls. However, reviewers rated this as low-certainty evidence, and because the product combined multiple ingredients, it’s impossible to say how much goat’s rue contributed on its own. No scientifically rigorous clinical trial has isolated goat’s rue and confirmed it works as a standalone lactation aid.
How People Take It
Goat’s rue supplements come in several forms. The dried leaves and flowering tops are the parts used medicinally. As a tea, the typical preparation is one teaspoon (about 5 mL) of dried herb steeped in a cup of hot water, taken twice daily. Liquid tinctures are also available, with common doses ranging from 1 to 2 mL taken three times a day. Capsules containing dried leaf powder are widely sold as well, often blended with other herbs in lactation support formulas.
There are no standardized dosing guidelines from any major regulatory body. The FDA lists goat’s rue in its dietary supplement database, but this classification means only that it appears in supplements on the market. It does not indicate that the agency has evaluated it for safety or effectiveness.
Safety and Toxicity Concerns
Goat’s rue is not a benign plant. Its leaves and flowering tops contain multiple guanidine derivatives, including galegine, which can cause blood sugar to drop. Anyone already taking diabetes medication or insulin faces a real risk of hypoglycemia if they also take goat’s rue, since both the herb and the medication push blood sugar in the same direction.
The plant’s toxicity is well documented in animals. In sheep, clinical signs of poisoning appeared at doses of just 0.8 grams of plant material per kilogram of body weight, and death occurred at 10 grams per kilogram. Rats fed 5 grams per kilogram of dried plant material survived without visible symptoms, but examination revealed significant liver and lung damage, suggesting those organs are the primary targets of goat’s rue toxicity. In mice, the average lethal dose of galegine sulfate (the isolated toxic compound) was 77.5 mg per kilogram of body weight.
Human toxicity data is limited, largely because people typically consume goat’s rue in small amounts as tea or capsules rather than eating the raw plant. Still, the guanidine content means the risk of low blood sugar is real, particularly for people who are fasting, have liver conditions, or take medications that affect glucose levels. The fresh plant is considered more dangerous than dried preparations, as drying reduces the concentration of some toxic compounds.
Why It Persists in Herbal Medicine
Goat’s rue occupies an unusual position. Its chemical compounds genuinely do affect blood sugar, which is proven by the existence of metformin. And its long folk history as a lactation aid keeps it popular among breastfeeding parents looking for natural options. But the gap between traditional use and clinical proof remains wide. The lactation studies that exist are old, small, and poorly designed. The blood sugar-lowering effects, while real, are better and more safely achieved through pharmaceutical metformin, which delivers a precise, controlled dose of a single well-studied molecule.
For people considering goat’s rue supplements, the practical reality is that you’re taking a product with plausible but unproven benefits, real risks of blood sugar disruption, and no standardized dosing. The combination of guanidine compounds in the raw plant is far less predictable than a pharmaceutical derived from those same compounds, which is exactly why metformin replaced the plant rather than the other way around.