The Food and Drug Administration has turned down an application by two drugmakers to produce an allergy drug combining the active ingredients of their popular Claritin and Singulair tablets. Schering-Plough/Merck Pharmaceuticals, a joint venture of Kenilworth-based Schering-Plough Corp. and Whitehouse Station-based Merck & Co., said late Friday that it had received a not-approvable letter from the FDA. Schering-Plough/Merck said it is evaluating the agency's response but did not provide further details. It filed an application for review of the proposed new drug in August 2007. The FDA decision does not affect the availability or continued use of either Singulair or Claritin. The prescription allergy and asthma pill Singulair is among Merck's best-selling products, with sales of $4.3 billion last year. Schering Plough's Claritin, formerly a prescription drug, is off-patent and is now sold over-the-counter.