This chapter is written in the question and answer format, which gives the description on clinical pharmacology. This chapter covers the pharmacogenetics, adverse drug reaction (ADR), drug interaction, drugs used in liver disorder, drugs use in kidney disorders, and drugs use in pregnancy. Drug metabolism that varies on genetic basis is often called polymorphic drug metabolism. Adverse drug reaction (ADR) is an unwanted or harmful reaction experienced following the administration of a drug or combination of drugs under normal conditions of use and suspected to be related to the drug. Drug interaction is a measurable modification (in magnitude and/or duration) of the action of one drug by prior or concomitant administration of another substance, including prescription, non-prescription (including complementary medicines) drugs, food, alcohol, cigarette smoking or diagnostic tests.