Eighteen hours after a 6-month-old boy in Bangkok started an antiviral for COVID-19, his mother noticed that his eyes had turned bright blue, most visibly in sunlight. The color came from his corneas, the clear domes at the front of the eyes, and it disappeared only after the drug was stopped.
The case, reported by pediatricians at Chulabhorn Hospital in Frontiers in Pediatrics, is the first instance of favipiravir-linked corneal discoloration in a child that the authors could find. Live Science revisited the case in May, describing the boy's eyes as normally dark brown. The drug behind it has since taken on a new role: Japan approved favipiravir in 2024 as the first drug anywhere for severe fever with thrombocytopenia syndrome, or SFTS, a tick-borne viral infection.
Eighteen Hours, Then a Color Change
The infant arrived after a day of fever and cough and tested positive for COVID-19 on an antigen test. Under Thailand's national guidelines at the time, favipiravir was the main antiviral for children with mild to moderate illness. He received tablets at 82 milligrams per kilogram on day one, followed by a pediatric syrup at 29 milligrams per kilogram per day on days two through five.
His mother spotted the blue tint at 18 hours. No discoloration appeared on his skin, nails or the lining of his mouth and nose. His COVID symptoms improved after three days of treatment, and the pediatrician advised stopping the drug because of the eye change. His corneas returned to their normal color on the fifth day after the medicine was stopped. The authors described him as the youngest known patient to receive favipiravir for COVID-19.
An ophthalmologist examined him two weeks after the infection using a handheld slit lamp. His corneas were clear, no blue pigment was found on the iris or lens, and nothing fluoresced under a cobalt blue filter. He could fix on and follow a light in every direction.
Favipiravir's more familiar side effects are different. Mild rises in uric acid, diarrhea and drops in neutrophils, a type of white blood cell, together account for about 20 percent of adverse events, the report noted. Blue corneas belong to a much smaller group of rare reactions that also includes temporary blurred vision and fluorescence of the eye's surface.
A Drug That Glows Under Ultraviolet Light
The Bangkok boy was not the first patient to show this effect. In 2021, doctors in Goa, India, reported in the Indian Journal of Ophthalmology that a 20-year-old man noticed bluish corneas on his second day of favipiravir for COVID-19, and the color returned to normal the day after he stopped. Other reports have described the eye's surface glowing under ultraviolet light, as well as fluorescent hair and nails under a Wood's lamp in patients taking the drug.
The Bangkok authors pointed to laboratory work showing that favipiravir tablets fluoresce under ultraviolet light, and to evidence that the drug's active metabolite in human plasma fluoresces in proportion to its concentration. Tablet ingredients such as titanium dioxide and yellow ferric oxide have also been proposed as contributors. The exact mechanism in the eye remains unproven.
The baby's color took far longer to clear than the adult's did. The authors suggested that age, dose and treatment length might matter, and that reduced urine output could have slowed clearance, since younger patients on favipiravir more often show rising uric acid. They presented that idea as plausible, not established.
The Drug Behind the Blue Eyes Has a New Job
Favipiravir, sold in Japan as Avigan, was first approved there in 2014 for novel or re-emerging influenza. Its record against COVID-19 was mixed, and the Bangkok report itself cites a randomized trial that found no demonstrated benefit in early mild to moderate illness.
In June 2024, Japan approved favipiravir for SFTS, according to researchers at Ehime University who led an early trial. In that uncontrolled, single-arm study of 23 analyzed patients, 17.3 percent died within 28 days, compared with case fatality rates of 27 to 31 percent in Japanese surveys. A later company trial by FUJIFILM Toyama Chemical reported mortality of about 13 percent, the Ehime team said.
Not everyone is convinced about the size of the benefit. In a comment in PLOS Neglected Tropical Diseases, Japanese researchers noted that national inpatient data showed untreated SFTS patients dying at rates of 17.1 to 18.8 percent, close to the trial's figure, though they cautioned that differences in how patients were selected make the comparison unreliable. The single-arm trial, they concluded, cannot pin down how much favipiravir reduces deaths.
Favipiravir is not approved by the FDA, and it is not sold in the United States.
Harmless in This Case, but Still Unmeasured
The child's corneas cleared completely, and his eye exam two weeks later was normal. Even so, the authors wrote that the long-term effects of this side effect have not been identified and that its frequency in children is unknown. They called for close safety monitoring when favipiravir is given to children who are still developing.
This is a single case report, and corneal discoloration appears to be rare. It does not show that favipiravir causes lasting eye damage. Parents who notice a sudden change in a child's eye color while the child is taking any medication should contact the prescribing clinician promptly rather than stopping the drug on their own.
Key Questions Answered
Why did the baby's eyes turn blue?
His corneas became discolored after he started favipiravir. The drug and its metabolites fluoresce, but how that produces blue corneas has not been proven.
Was the color change permanent?
No. His corneas returned to normal five days after the drug was stopped, and an eye exam two weeks later was normal.
How common is this side effect?
Its frequency is unknown. Only a handful of cases have been published, and this is the first the authors found in a child.
What is favipiravir used for now?
In Japan, it is approved for certain influenza outbreaks and, since June 2024, for SFTS, a tick-borne viral infection.
Is favipiravir available in the United States?
No. It is not FDA-approved and is not sold in the United States.
What should parents do if a child's eye color changes on a medication?
Contact the prescribing clinician promptly. Do not stop the drug without medical advice.
