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Singulair (montelukast) is a drug prescribed to patients suffering from asthma or severe allergies that can provide symptom relief and help them manage their condition. It is a very successful drug for its manufacturer, Merck & Co. Singulair received the U.S. Food & Drug Administration’s (FDA) marketing approval in 1998; by 2004, Singulair was doing $2.62 billion in sales.