Essential drugs are those drugs, which satisfy the priority healthcare needs of the population. In India, its current edition is 18th National List of Essential Medicines. It was revised in 2013 and contains 406 drugs. Orphan drugs or biological products for diagnosis/treatment/prevention of a rare disease or condition or a more common disease for which there is no reasonable expectation that the cost of developing and marketing it will be recovered from the sales of that drug. Therapeutic index is a measure of the safety of the drug. Graphically, it is the gap between the therapeutic effect DRC and the adverse effect DRC. Enzyme induction refers to increase in the synthesis of microsomal enzyme protein due to interaction with another substance leading to increased metabolism of inducing drug itself and /or other drugs. Receptor antagonism, partial agonist and inverse agonist, drug synergism, pharmacovigilance, first-pass metabolism, plasma protein binding of drugs, plasma half-life, loading dose, drug treatment of glaucoma, levocetrizine, sodium cromoglycate, lignocaine, etc. are the other topics on which short notes are given in this chapter.