It can be understood by the example given below. Atropine is non specific antagonist of muscarine receptors. It can be used in the management of bronchial asthma by viue of its M3 blocking action (bronchodilation). But it can cause tachycardia and adverse effects due to blockade of cardiac M2 receptors. A more specific drug like tiotropium bromide(selective M3 antagonist) is less likely to cause these adverse effects. Refer Katzung 10/e p29