Blue flag (Iris versicolor)

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15. Safety

Key points. Blue flag is an emeto-cathartic drug with a narrow comfortable range. Harm rises with dose and is worse with fresh root. Felter (1922) wrote that iris "is capable of causing gastro-enteritis resulting in death" [19]. That is a historical statement with no case details, but it rules out self-dosing above the practitioner range. AIP Medicine advises against use in pregnancy, while breastfeeding, in children outside professional care, and in active bowel inflammation or bile duct obstruction.

Adverse effects in use. No trial has recorded adverse events, because no trial exists (section 13). The practitioner texts record gastrointestinal upset at or above the top of the dose range. MediHerb advises monitoring sensitive individuals and patients taking the high end of the dose [13]. Barnes lists nausea, vomiting and irritation of the gut and eyes, put down to the fresh root and to furfural in the volatile oil [8]. Duke records the same from Barnes’s first edition, and adds practitioners’ reports that fresh-root tinctures are much stronger than dried-root ones [16]. Felter records "a more or less persistent belly-ache and mild catharsis" even at "moderately full therapeutic doses" [19]. The historical texts describe large doses as causing acid vomiting, watery purging with intestinal burning and colic, salivation, facial neuralgia and "considerable prostration" [11,19]. Barnes, weighing the absence of data and the known toxicity of related species, states that "the use of blue flag is not recommended" and that it "may not be suitable for internal use" [8]. That is an overseas pharmacist’s judgement and is recorded as such. Grade: practitioner and historical observation; no controlled data.

Case reports. No published case report of harm from blue flag in a human was found in either screen (section 13). The USDA guide of 1907 records "cases of poisoning among children" who mistook blue flag for sweet flag, and gives the name "poison-flag" for that reason, without case details [20]. Felter’s fatal gastro-enteritis is likewise given without a case [19]. Causality cannot be assessed for either.

Toxicology. No study of the whole drug in animals was found. Barnes reports a rat oral LD50 for furfural of 127 mg/kg, and describes furfural as irritant to mucous membranes, causing lachrymation, eye inflammation, throat irritation and headache; whether the irritancy of blue flag comes from its volatile oil "has not been established" [8]. The volatile oil is about 0.025 per cent of the drug [8], so the furfural LD50 cannot be turned into a dose limit for the herb. The rhizome extract was cytotoxic to human fibroblasts at 96 mg/L in vitro [24]. King’s records that animals dying after ingestion showed "marked congestion of the gastric and intestinal tissues" [11]. Grade: laboratory, animal and historical data.

Contraindications and cautions.

The pregnancy statements come from different frameworks. The AHPA class 2b and the Mills and Bone category B1 appear side by side in Hechtman’s Appendix 2, taken from Romm (2009) [14]. Duke gives AHPA classes 2b and 2d [16]. Barnes also lists blue flag among herbs best avoided in pregnancy, for its irritant oil [8]. King’s records that an "iridin" pill was once given in Edinburgh for the vomiting of pregnancy [11]. That is history, and it says nothing about safety. The Medicine view is to avoid blue flag in pregnancy and while breastfeeding. No source reports harm in either. The advice rests on the missing data and on a purgative, mucosal-irritant action that could harm a pregnancy or reach an infant.

Herb-drug interactions. None documented. Barnes and MediHerb record none [8,13]. Stockley’s Drug Interactions (2008) and Lam, Huang and Hall (2006), both held in the AIP reference library, have no entry for blue flag or Iris [41,42]. MediHerb refers users to its herb-drug interaction chart, which was not among the sources read [13]. Theoretical interactions (AIP Medicine interpretation, untested):

• A cathartic dose could shorten gut transit and reduce the absorption of oral drugs taken at the same time. This matters most for drugs with a narrow margin (for example levothyroxine, lithium, digoxin, warfarin and antiepileptics) and for oral contraceptives if vomiting or diarrhoea occurs.
• Repeated purging could lower potassium and add to potassium loss from diuretics, corticosteroids or stimulant laxatives. A low potassium level increases the toxicity of digoxin.
  Grade: theoretical for both. There is no clinical report and no pharmacokinetic study.

Older people. No data. The risk of fluid and potassium loss from purging is greater in older people and in anyone taking the drugs listed above.

Overdose. The recorded picture is emesis, purging with colic, salivation and prostration [11,19], with a historical report of fatal gastro-enteritis [19]. Management is supportive, with attention to fluid and electrolyte loss. In an emergency in Australia call 000, or the Poisons Information Centre on 13 11 26.

TGA safety information. TGA safety alerts and recalls: on 30/09/2026 the addresses of all 767 safety-alert pages and all recall and market-action pages listed on the TGA website were searched for Iris, blue flag, Acorus and calamus. None matched. The search picked up its controls (the kava, turmeric, withania and garcinia alerts) [43]. An alert on a multi-ingredient product that did not name blue flag in its title would not be found this way. The TGA Database of Adverse Event Notifications (DAEN, medicines) was searched through its interactive portal in October 2026, for reports received from 01/01/1971 to September 2026. "Iris versicolor" and "blue flag" matched no medicine and no report. "Iris" matched three medicines, none containing blue flag (an Irish moss cough syrup and two monoclonal antibodies), and "versicolor" matched six, all containing the mushroom Trametes versicolor. The control, Cimicifuga (black cohosh), matched 14 medicines with 36 reports [43]. DAEN lists a medicine only under the ingredient names it was reported with, so a report on a product that did not name blue flag would not be found.

Confusion with sweet flag. Sweet flag (Acorus calamus) is Schedule 10 for human therapeutic use in Australia [4], and the two rhizomes have been mistaken for each other [20]. A misidentified lot would be a legal and a safety problem. The identity tests of section 10 exclude it.

Referral. Anyone thinking of taking blue flag, especially in pregnancy, while breastfeeding, for a child, with gallstones or bowel disease, or with other medicines, should consult a degree-qualified herbalist with competencies and adequate training in pharmacognosy.