Explained: How An Infant’s Dark Brown Eyes Turn Blue After COVID-19 Antiviral Treatment

The unusual effect was first reported in December 2021, when the corneas of a 20-year-old man with brown eyes turned blue for a day after he took favipiravir.

But in the summer prior to that report, another group of doctors reported that a man came to their hospital with a UV light to show that the surface of his eyes glowed fluorescent after he took favipiravir. A 2022 case report described flecks of fluorescence that appeared in the whites of three people’s eyes, as well as in their nails and some teeth, after they took the drug, LiveScience reported.

Most recently, doctors reported a peculiar case of eye discolouration in a 6-month-old boy. According to a report published in April in the journal Frontiers in Pediatrics, the boy was taken to a hospital in Thailand after he developed a fever and cough. After testing positive for COVID-19, he was given favipiravir tablets and also a syrup containing the drug.

Just 18 hours after the infant took the drug, his mother noticed that his eyes, usually dark brown, shone blue in the sunlight. Upon examining the child, doctors discovered a buildup of blue pigment in both corneas.

The boy received favipiravir for three days, after which his COVID-19 symptoms improved. His doctor then halted the treatment due to the strange colour change in the infant’s eyes. Five days after the treatment was stopped, the boy’s eyes returned to their usual colour.

“Usually the color of the eye is determined by the iris, not the cornea and is determined by the amount of pigment that is present in the iris from birth,” Dr Vik Sharma, an eye surgeon at the LondonOC clinic in the U.K. who was not involved in the boy’s case, told Live Science in an email.

In the new report, the boy’s doctors wrote that this fluorescence “may be due to the drug, its metabolites, or additional tablet components such as titanium dioxide and yellow ferric oxide.” Favipiravir tablets have been found to be fluorescent under UV light in the lab, they noted, so it may be that the drug’s fluorescent components end up accumulating in different tissues.

When an eye doctor examined the boy’s eyes two weeks after he recovered from COVID-19, there were no signs of problems with his vision. However, it remains unclear whether there might be any longer-term effects of the temporary change in his eye colour, the doctors wrote in the report.

“More work is needed to determine the exact cause [of the eye discoloration] and any long-term effects,” Sharma was quoted as saying.

Factors such as people’s age, treatment duration, and drug dosage may influence their odds of developing the rare side effect and how long it takes for the eye discoloration to disappear, the report authors wrote. But again, because this strange effect has only been reported a few times, it’s unclear exactly why or how favipiravir discolors some individuals’ eyeballs while leaving most people’s eyes their original hues.