If henna is absorbed into the skin or ingested, it can cause acute anemia and even death for people who lack the G6PD enzyme. There are several cases in the medical literature of babies or young children who have had to be hospitalised after henna was applied to their skin.
As far back as 1996, a report in Pediatrics described placing lawsone, a chemical found in henna, together with red blood cells from people with and without G6PD for two hours at 37C, or body temperature. Lawsone was found to cause haemolysis, or destroy the cells, for those with G6PD deficiency. This in turned released bilirubin, whose colour is seen in jaundice, or it can cause anemia, which manifests as pale skin. The report suggested that henna applied on the skin of babies may have caused jaundice for areas where G6PD deficiency is common (parts of Africa, Asia and the Mediterranean).
Another 1996 report from the Annals of Tropical Paediatrics describes 15 G6PD-deficient male newborns admitted to Al-Jahra Hospital in Kuwait over 10 years with acute haemolysis a few days after applying henna dye over the body. According to the report, applying henna on newborns is a Bedouin tribal practice to celebrate the arrival of the first-born boy.
In 2001, Archives of Disease in Childhood describes haemolytic crises in four G6PD-deficient children from Oman and the UAE after henna was applied to their skin: a female newborn who recovered after a blood transfusion, a male two-month-old who died after henna was applied to his palms and his soles despite a transfusion, and two preschool children, a boy of three who started showing symptoms after three days, and a girl of four, who became ill after two days. Both survived. The study notes that other causes had been ruled out, making it a strong possibility that these incidents had been caused by henna alone.
In 2005, the Iranian Journal of Pediatrics reported that that a seven-year-old boy had been admitted with pallor**, jaundice and hemogluinuria*** after having henna on his skin.
In another case a teenage girl was found to have partial G6PD deficiency after she ingested a herbal preparation containing henna and suffered severe haemolytic anemia.
The problem tends to show up two to three days after the henna application, as passing red urine, pale skin and vomiting. If there is a family history of G6PD deficiency and someone becomes ill several days after coming into contact with henna, it is best to seek medical help immediately. And while doing henna may be an uneventful process for the majority of people, those who suspect they may be G6PD-deficient should get tested before trying any henna at all.
*People with G6PD deficiency may also suffer similar problems after inhaling the gas from mothballs and from eating broad (fava) beans. A list of drugs to avoid is here. The same site has a list of foods to avoid.
**Unusually pale skin.
**The urine is abnormally dark because very high levels of hemoglobin, the oxygen-carrying component of blood cells, are excreted into the urine.