The Food and Drug Administration is warning doctors, parents and patients about the potential for eye disorders linked to Kalydeco, a groundbreaking treatment for cystic fibrosis approved earlier this year.

The FDA says the warning is based on cases of cataract seen in juvenile rats studied by the drug’s manufacturer, Vertex Pharmaceuticals. The study was designed to show that Kalydeco is safe for children younger than 2 years old. Cataract is a clouding of the eye’s lens that can disrupt vision

Vertex is adding information about the study to Kalydeco’s label, though the FDA states: “There is uncertainty about the relevance of this risk to children, since there are differences in eye development between humans and rats.”

The FDA is requiring Vertex to conduct a study of vision in children 11 and younger who are taking Kalydeco. The children will be followed for at least two years and will receive eye examinations every six months.

Kalydeco is the first medication that affects the underlying cause of cystic fibrosis, an inherited disease that causes sticky mucus buildup in the lungs and other organs, leading to infections and death in young adulthood. The twice-a-day pill affects a specific genetic defect found in a small subset of patients who have the disease. The FDA approved the drug in March for cystic fibrosis patients 6 years old and up.

Shares of Cambridge, Mass.-based Vertex Pharmaceuticals Inc. fell 75 cents to $53.34 in afternoon trading.