The twin requirements of respiration (ventilation and perfusion) result in the lungs both being exposed to the external environment and the internal environment through the bloodstream. Many of the drugs that people use as anesthetists are metabolized in the lungs including sympathomimetics, antihistamines, opiates and local anesthetics. Drugs used in the treatment of asthma and bronchospasm are bronchodilators, leukotriene antagonists, and magnesium sulphate. Opiates, propofol, thiopentone and muscle relaxants are used during anesthesia on the respiratory system. Mucolytics are prescribed in order to facilitate expectoration by decreasing the viscosity of sputum. They have been shown to benefit some patients with chronic obstructive airways disease and chronic cough with a reduction in exacerbations of the condition. Patients with respiratory disease are likely to be particularly sensitive to small decreases in respiratory muscle function secondary to residual muscle weakness.