Regeneron’s Eylea Has Mysterious Rise in Side Effect Reports

Doctors say they’re seeing an unexplained increase in a rare, frightening side effect in patients getting Regeneron Pharmaceuticals Inc.’s top-selling product, the blockbuster eye drug Eylea.

After the drug is injected into the eye, a small number of people have a sharp decline in sight, experience pain, and can panic. While the effect is treatable and doesn’t lead to long-term vision loss, a professional society of eye physicians has warned members to look out for the condition.

The company said it’s closely monitoring the reports, which are a known complication of Eylea and its rivals. In the past, occasional clusters ...