影响生物过程的任何分子。更严格地说是药学活性可以和化学结构相关联的分子。历史上，药物是从自然发生的真菌、植物和其它动植物中识别并提取的，对于特定疾病的导靶作用几乎没什麽方向性。然而逐渐地，现代药物是通过准确导向分子水平的特定疾病状态发现的，药物可以是基因、基因的蛋白产物（如重组胰岛素或促红细胞生成素）或设计制造及修饰的小分子以调节特殊的疾病过程。无论新药开发过程如何，所有的药物都经过一个漫长的临床前和临床检验；第一个临床前检验是测试药物在动物模型中的存活力和药用（通常是鼠或更高级的灵长类动物）；第二次检验测试在人体中的安全性、药效和剂量。这些检验通过后（通常需要3-5年，花费5千万到2亿美金）最终要向本国政府授权机构提出申请（在美国是食品药物管理委员会）。只有当该机构检验了药物测试的数据并认为它具有良好的治疗效果而没有严重的副作用，这种药物才被允许制造和销售。

Any molecule that affects a biological process. More strictly, a molecule whose pharmacological activity can be correlated with its chemical structure. Historically, drugs were identified and extracted from naturally occurring fungi, plants, and other flora and fauna, with little direction given to the targeting of a particular disease. Increasingly, however, modern drugs are being discovered through the precise targeting of a particular disease state dissected at the molecular level, and the drugs may be genes, the protein products of genes (such as recombinant insulin or erythropoetin), or small molecules created by design or diversity to modulate a specific disease process. Irrespective of the process of novel drug discovery, all drugs must undergo a lengthy process of preclinical and clinical review; the first preclinical review tests the viability and usefulness of the drug in model organisms (usually mice, rats or higher primates); and the second review (clinical trials, split into four phases) tests the safety, efficacy and dosage of the drug in humans. The culmination of these reviews (which typically take 3-5 years and can cost from $50-$200 million) is a submission to the issuing authority of the host country’s government (in the U.S, Food and Drug Administration.). Only once this authority has reviewed the data on the drug and agreed that it offers therapeutic benefit without serious side effects is the drug approved for manufacture and sale.