Drugs exert their therapeutic effects by interacting with receptors, enzymes, or ion channels that are present throughout the human body. The strength and duration of the interaction between a drug and its target receptor are characterized by the selectivity and specificity of the drug. Selectivity refers to a drug's strong preference for its intended target over other targets. For instance, isoprenaline, a non-selective β-adrenergic agonist, interacts with both β₁- and β₂-adrenergic receptors in the heart and smooth muscle tissues, respectively. Due to its lack of selectivity, it can stimulate receptors for both cardiac and bronchial effects. The nonselective nature of isoprenaline makes it less ideal for treating asthma. On the other hand, salbutamol is a selective β₂-adrenergic agonist that is commonly used to treat asthma. Its selectivity for β₂-adrenergic receptors allows it to specifically target the smooth muscle cells in the bronchioles, causing bronchodilation and relieving asthma symptoms.

Specificity describes the extent to which a drug produces only the desired therapeutic effect without causing any other physiological changes. A drug with high specificity exhibits a strong drug–receptor interaction, ensuring targeted action and minimal side effects. In contrast, drugs with low specificity may produce unintended consequences due to their weak drug-receptor interaction. For example, amiodarone, an antiarrhythmic drug, demonstrates low specificity, affecting multiple ion channels and causing various side effects. On the other hand, omeprazole, a proton-pump inhibitor, shows high specificity by selectively inhibiting gastric acid secretion without impacting other physiological processes.