Blue flag can cause nausea, vomiting, and loose stools.
– Ellingwood F. — American Materia Medica, Therapeutics and Pharmacognosy, 11th ed. — Eclectic Medical Publications, 1998, 312-3.

Blue flag should only be taken on the advice of a physician or herbalist trained in its use. Blue flag is unsafe for children or for use during pregnancy or lactation. Use of blue flag as a cathartic laxative is not recommended.
– McGuffin M, Hobbs C, Upton R, Goldberg A — American Herbal Product Association’s Herbal Safety Handbook. — CRC Press, 1997, 64.

There have not been extensive studies on the use of blue flag.