Gefapixant, a P2X3 receptor antagonist, has been advocated to be used for the treatment of refractory and unexplained chronic cough. A study by Smith J.A. et al evaluated its efficacy contrasting placebo after 12 weeks of treatment for refractory chronic cough or unexplained chronic cough. 

Patients suffering from refractory or unexplained chronic cough lasting 1 year or more, with no radiographic chest abnormality, and 40 mm or more on a 100-mm cough severity visual analogue scale at the time of enrolment were included in the study. 

Patients were either given a placebo or one of three doses (7·5 mg, 20 mg, or 50 mg) of oral gefapixant B.D. for 84 days. They were assessed on days 1, 28, 42, 56, 70, 84, and 85. 

The observations of the study were-

At 12 weeks, 

Mean awake cough frequency was-

• 18·2 coughs/h with placebo,
• 14·5 coughs/h with 7·5 mg gefapixant, 
• 12·0 coughs/h with 20 mg gefapixant, 
• 11·3 coughs/h with 50 mg gefapixant. 

The most common adverse event- Dysgeusia, reported in

• 5% patients given a placebo, 
• 10% given 7·5 mg gefapixant,
• 33% given 20 mg gefapixant, and
• 48% given 50 mg gefapixant. 

Thus, they formulated that administration of gefapixant at 50 mg twice daily dosage can significantly reduce cough frequency in patients with a refractory chronic cough or unexplained chronic cough after 12 weeks of treatment compared with placebo. 

Source: Smith JA, Kitt MM, Morice AH, Birring SS, McGarvey LP, Sher MR, Li YP, Wu WC, Xu ZJ, Muccino DR, Ford AP. Gefapixant, a P2X3 receptor antagonist, for the treatment of refractory or unexplained chronic cough: a randomised, double-blind, controlled, parallel-group, phase 2b trial. The Lancet Respiratory Medicine, 2020. https://doi.org/10.1016/S2213-2600(19)30471-0