The rhizome, or underground stem, of the blue flag (indicating its showy blue flowers) is used medicinally. Blue flag and closely related species (particularly Iris missouriensis, western blue flag) grow across North America.

Based on Native American traditions, Eclectic physicians (19th century doctors who relied on herbs) and herbalists used blue flag for a number of conditions. Of note was its use as a nonspecific immune enhancer, as a laxative, and to detoxify the intestinal tract.¹ Topical application of fresh, sliced rhizomes to the sores of impetigo (a common bacterial skin infection in children) has been recommended by herbalists.² Traditional herbalists have used blue flag to treat poor digestion characterized by fat malabsorption.

The resinous fraction of blue flag contains numerous phenolic glycosides. Traditional herbal texts suggest these constituents stimulate the parasympathetic nervous system, leading to production of bile, saliva, and sweat.³ However, modern clinical trials have not confirmed these effects for blue flag.

Herbalists sometimes recommend up to 10 drops of tincture of the dried rhizome be taken three times per day.⁴ The tea form is unlikely to be effective, since the active compounds in blue flag are not water soluble.

Blue flag can cause nausea, vomiting, and loose stools if too much is taken.⁵ People should not exceed the recommended amounts. Fresh rhizome should only be applied topically and never taken internally, since it can irritate the mouth⁶ and is much more likely to cause nausea and diarrhea. Blue flag should only be taken on the advice of a physician or herbalist trained in its use. Blue flag is unsafe for use during pregnancy or breast-feeding. People should not give blue flag to children.