Sometime in 2022, a mother in a Bangkok hospital noticed something that stopped her cold: her 6-month-old son’s dark brown eyes had shifted to a striking bluish-purple. He had been admitted with a fever and cough, tested positive for COVID-19, and started on a standard antiviral — all routine. But less than 24 hours into treatment, his corneas had turned a color no doctor had ever recorded in a child.
No other part of his body showed any discoloration. His skin, nails, and hair looked completely normal. Only his eyes had changed — and no one in the room had an immediate explanation.
A routine prescription with an extraordinary result
The drug at the center of this case is favipiravir, a broad-spectrum oral antiviral developed to target RNA viruses — a category that includes influenza, Ebola, certain enteroviruses, and coronaviruses. Doctors began using it against COVID-19 in 2020, and by 2022 it had become a relatively standard option in some treatment settings. For a 6-month-old with a confirmed positive test, prescribing it wasn’t unusual.
The plan was straightforward: favipiravir tablets on the first day, followed by a liquid formulation for the next four days.
Then, roughly 18 hours in, his mother looked at her son’s face and noticed his eyes had changed color. His corneas — the clear, dome-shaped surfaces at the front of each eye — appeared bluish-purple in sunlight. No discoloration anywhere else. Not his skin, not his nails, not his hair. Just his eyes.
Physicians reviewing the case later confirmed what they suspected in that moment: this was the first documented instance of favipiravir-induced corneal discoloration in a pediatric patient. The drug had produced an effect in this child that had never been recorded before.
Not entirely without precedent
The Bangkok doctors had one prior case to draw on — and it came from Goa, India, in 2021. Physicians there described a 20-year-old man who developed a bluish tinge in his corneas after starting favipiravir for COVID-19. It was the first time the side effect had been documented anywhere. When he stopped the drug, the discoloration disappeared the next day.
That same month — January 2021 — a separate report emerged from Istanbul. Four female COVID-19 patients who had taken favipiravir developed a glowing blue-green tint in their hair and fingernails. The color wasn’t visible under normal light; it only appeared under a Wood’s lamp, a device that uses long-wave ultraviolet light to detect fluorescence.
Later that year, Turkish researchers went further, documenting fluorescence in favipiravir tablets themselves — both whole and dissolved in water. The infant’s doctors cited this finding directly, calling it strong evidence of a link between the drug and fluorescent changes in body tissues. The Bangkok case extended this pattern to its youngest patient on record and added a new dimension: what happens when an infant’s still-developing system processes the same compound?
Why does favipiravir change eye color?
The leading hypothesis centers on how the body metabolizes favipiravir. As it breaks down the drug, it may release fluorescent chemical compounds that accumulate in the cornea. The cornea’s transparency — the quality that makes it function as a window into the eye — may also make it a particularly visible site for this kind of buildup, even when other tissues appear unaffected.
There’s a secondary mechanism worth considering. Favipiravir is known to elevate uric acid levels, which can reduce how efficiently the kidneys excrete certain compounds through urine. If fluorescent byproducts aren’t being cleared quickly, they may linger in tissues far longer than they otherwise would.
In an infant, that effect could be more pronounced. Babies urinate less frequently than adults, and their metabolic and renal systems are still maturing. That combination — slower clearance, developing physiology — may help explain why the discoloration in this case took longer to resolve than it did in the adult patient from Goa.
Recovery and what doctors learned
The infant’s COVID-19 symptoms began improving after three days on favipiravir. On day five, a pediatrician made the call to discontinue the drug. Over the following five days, the blue tint in his corneas gradually faded.
Two weeks after discontinuation, an ophthalmologist examined his eyes directly. The corneas were clear. No bluish color remained, and no fluorescence was detected. By every measure, his eyes had returned to normal.
The case raised a practical question doctors couldn’t fully answer: why did the discoloration take several days to clear in the infant, when the 20-year-old in Goa recovered almost overnight? The treating physicians proposed that patient age and medication dosage were likely factors, though the precise mechanism remains uncertain. Known adverse effects of favipiravir include diarrhea, elevated uric acid, and reduced neutrophil counts — the white blood cells central to fighting infection. Corneal discoloration is rare, but it’s now documented across multiple patients in multiple countries, spanning different ages and presentations.
What comes next for this rare side effect
This case was published in Frontiers in Pediatrics in 2023, adding a formal record to a still-small body of literature. Favipiravir continues to be used in various countries as a COVID-19 treatment, and in some regions it remains a first-line option when other antivirals are unavailable or unsuitable.
For clinicians prescribing it to young children, this case offers something concrete: a documented precedent. Parents and caregivers should know that eye color changes, while alarming in appearance, resolved completely in both pediatric and adult cases once the drug was stopped.
Exactly how favipiravir produces fluorescent compounds in human tissue — and which patients are most susceptible — remains an open question. As the drug stays in use, more cases may emerge. Each one will add another data point to a phenomenon that, until 2021, had no place in the medical literature at all.