In-depth analysis of the structure of the drug and its subsequent modification yield drugs with high affinity and increased receptor specificity with an improved pharmacokinetic profile. Modification of the parent molecule of catecholamines, called β-phenyl-ethyl-amine, provided orally active adrenergic bronchodilators, long-acting β2-adrenergic agonists, COMT-resistant catecholamines, and isomerism-based increased potency in adrenergic agonists. Similarly, alteration of the cyclo-pentano-perhydro-phenanthrene ring of steroid molecules delivered androgen, progestins, and estrogen with low first-pass metabolism, long-acting injectable steroids, and corticosteroids with negligible mineralocorticoid activity. In addition to this, manipulation of the structure of morphine resulted in a plethora of “opioid agonists and antagonists” that are used in clinics for various conditions. Minor alteration of “spacer” in the structure of antihistaminic molecules resulted in “nonsedative” antihistaminics. Therefore, SAR plays a vital role in the drug development process which ultimately determines the “success” or “failure” of the drug.