WASHINGTON (AP) — Federal health regulators say an experimental combination drug for cystic fibrosis developed by Vertex Pharmaceuticals Incorporated helped improve breathing in patients with the deadly inherited disease.

But the Food and Drug Administration says in an online review that the benefit was small and it is unclear the addition of a second drug ingredient adds to the pill’s effectiveness.

About 30,000 Americans have cystic fibrosis, which causes sticky mucus buildup in the lungs and other organs, leading to infections, digestive problems and early death.

Vertex’s experimental drug, which would be sold as Orkambi, is the company’s follow-up to its breakthrough treatment Kalydeco, which the FDA approved in 2012. Orkambi combines the active ingredient in Kalydeco and a new drug, lumacaftor.

But FDA scientists say it’s unclear that lumacaftor “contributes any added benefit.”