Side effects of a drug arise due to interactions of the drug to molecules other than the target. These effects of the drug can be minimized by its high:

It can be understood from the example given below: Atropine is a non specific antagonist of muscarinic receptors. It can be used in the management of bronchial asthma by virtue of its M3 blocking action (bronchodilatìon). But it can cause tachycardia as an adverse effect due to blockade of cardiac M2 receptors. A more specific drug like tiotropium bromide (selective M3 antagonist) is less likely to produce this adverse effect.