New drug development involves animal toxicity studies followed by clinical trial in humans. Potentially useful compounds are studied in animals to evaluate desired effects and toxicity. Animal toxicity studies are done to see whether the chemical is safe or not. Compounds that appear effective and safe are candidates for human studies. For all new drugs, it is mandatory to do toxicity studies in animals. After that the drug enters various phases of clinical trials. The investment to produce one marketed drug is approximately US$800 million and 12-15 years. The idea starts from folklore or screening of natural products. But nowadays drugs are designed on computer keeping in view the target e.g. agonists and antagonists are synthesized for receptors.