Merck & Co Inc. has had a setback with its chronic cough drug gefapixant after the FDA rejected its NDA, a decision that also briefly hit the share price of rival Bellus Health Inc. before it regained market traction late Jan. 24. The FDA had been reviewing gefapixant since March 2021, but the regulator is now asking for additional information related to measurement of efficacy in a dreaded complete response letter. Merck said the response was not related to the safety of gefapixant, a P2X3 receptor antagonist, under development for treatment of refractory chronic cough (RCC) or unexplained chronic cough in adults. Shares in Merck (NYSE:MRK) ticked down 1.4% to $78.86 while Bellus Health shares (NASDAQ:BLU) rose by 1.8% to $5.66.