Pharmacology is the study of how drugs affect biological system and how the body responds to the drug. The discipline comprises the sources, chemical properties, biological effects and therapeutic uses of drugs. The effects can be classified as either therapeutic or toxic, depending on a number of factors. Pharmacologist specialists have an interest in therapeutics, whose major focus is on the effects of drugs and other chemical agents used to alleviate or treat diseases, or toxicology, which involves the study of adverse/toxic effects of drugs and other chemical agents. Pharmacology has two main branches; Pharmacokinetics and Pharmacodynamics. In summary, pharmacokinetics is what the body does to the drug and pharmacodynamics is what the drug does to the body. A focal point of pharmacology has been the advancement of knowledge about the cellular response as a result of drug interaction and action. The knowledge and understanding of how drugs interact with cellular targets allows pharmacologists to come up more selective drugs with better efficacy and fewer undesirable side effects.