Drug is any natural, synthetic or chemical substance that, when it incorporates into a living body, should show positively effects on that body, and it will also help in the diagnosis, treatment, minimize infection and supply of a deficiency It is the body’s mechanism to be on the correct path of function, or to prevent illness or relieve pain in any part of that body or to balance a specific organ in the body. It also called medicine.In general, the drugs act by interacting with receptors on the surface of cells or enzymes in cells. Receptor and enzyme molecules have a specific three-dimensional structure; this structure allows only certain substances that fit exactly to attach to it, this technique is often referred to a model called ”a lock and key model”.The development of drugBefore discovering a new specific drug, scientists are working to understand the disease and understand how to treat it as well as possible and explain the underlying cause of the disease. They seek to understand how genes are transformed, how they affect the proteins they code for, how these proteins cooperate with each other in living cells and how they interact with each other, how these cells alter the specific tissue, and how they interact Disease on the entire tissue is patient. This knowledge is the basis for the treatment of the disease or the health problem.There are 330,000 human genes that provide the molecular therapists and scientists, different options to find new medicines, by understanding how genes work can be the key to select the correct genes that are considered to be targeted for the drug discovery process. 2Select a molecule to target with a drug:If the fundamental cause of a disease is sufficiently understood, the biomedical scientists and pharmacists might be able to select a target for a specific new drug, while these drugs act on either the cellular or genetic composition of the body, known as “targets.” The target that showing an actual association with the disease is generally a single molecule, such as a gene, protein, polysaccharides, lipids or nucleic acid involved in a particular disease.The identification and characterization of the most appropriate target within a disease pathway can be one of the problems or the challenging that could associate with the discovery of new drugs.After selecting a correct potential target, scientists must demonstrate that the target is actually involved in the disease, they should understand how it can respond to a drug, so selecting the correct targets is very useful for developing the new treatment for disease compare each drug goal with others based on their association with a particular disease.Phases of clinical trials of the drug:In the discovery of a drug must be organized and supervised in a specific phase of clinical trials of drugs, there are many companies around the world are ignoring or discontinuing their research on drugs when the drug fails to achieve its purpose or because it is dangerous to be used due to its side effects for example. most of the researchers trying to carry out their researches in laboratories and animal studies to find suitable compounds that are safe to be tested for human, but before these companies begin their clinical trials and research, certain approval must be approved by the National Health Agency of the country that the company is actually located and working in, the pharmaceutical company should make send a request for its new drug investigation.In clinical trials of drugs that may take six or seven years, drug compounds should be administered to humans by test it on group of people in three phases; in order to study the benefits of the drug and trying to improve it, as well as to determine the risk and the side effects and minimize it to the safest level. All information and analysis data must be kept and studied well to ensure that the drug test is safe and acceptable according to the regulations of the health agency and it should be described in a new drug monograph.The clinical trials phases consist of 4 phases:In Phase 1:The tests are usually conducted in healthy volunteers; these tests were conducted in specialized that has received approval from the health agencies or authorities. These studies have two major objectives: First, it is to ensure that the toxicity results obtained during pre-clinical development are comparable to those obtained in humans. This help to determine the maximum dose of the drug development tolerated in humans. Second, it is the measurement, using pharmacokinetic studies, the fate of the drug in the body according to its mode of administration (absorption, distribution, metabolism, and excretion).In Phase 2: The trials are designed to determine the optimal dosage of the product in terms of safety and efficacy in a limited and homogeneous population of people which several hundreds of volunteers should try the drug compounds test on. Drug interactions and pharmacokinetics sometimes studied at this early stage. In phase 3:The tests are larger and conducted on several thousand representatives of the population of volunteers in which the treatment is for patients. These comparative tests, in which the drug is compared to developing an effective treatment already designed and marketed or so the treatment would be without pharmacological activity. This comparison is most often and double-blind, so the treatments are randomly assigned without the patient and the doctor in charge of monitoring are aware of what they award experienced, these tests are intended to demonstrate the therapeutic value of the drug and to evaluate its benefit and risk ratio.In phase 4:in this stage the drug would be ready and approved to sale out in the markets and pharmacies and then to the costumers but also the company that produced the drug has many objectives to be achieved in this phase such as comparing the new drug to another similar drug which actually in the market and used by people, check the long term of the drug efficiency and its effect on patient’s life and also, the company can advise to take the drug off the market and restrict it from being used by people depend on the drug or the product studies.