Merck & Co said on Monday the US Food and Drug Administration declined to approve its experimental drug for the treatment of chronic cough and sought additional information on the treatment's effectiveness.

Merck said it would meet with the agency to discuss the next steps on the oral drug, gefapixant.

Shares in the company were down about 0.5% at $79.6 before the bell.

If approved later, gefapixant would become the first such remedy for patients who have a persisting cough after the treatment of underlying health conditions such as asthma or if they have an unexplained chronic cough, Merck said.

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Last week, Japan's health regulator approved gefapixant tablets for adults with refractory or unexplained chronic cough, to be marketed under the brand name Lyfnua.

The FDA's decision was not related to the drug's safety, Merck said.