Marex Syrup is a combination of four medicines: Ambroxol, Guaifenesin, Menthol and Terbutaline. Ambroxol is a mucolytic which thins and loosens mucus (phlegm), making it easier to cough out. Guaifenesin is an expectorant which works by decreasing the stickiness of airway secretions and helps in their removal from the airways. Menthol is an organic compound which produces a sensation of coolness and relieves minor throat irritation. Terbutaline is a bronchodilator which relaxs the muscles in the airways and widens the airways.Together, they make breathing easier.