影響生物過程的任何分子。更嚴格地說是藥學活性可以和化學結構相關聯的分子。歷史上，藥物是從自然發生的真菌、植物和其它動植物中識別并提取的，對于特定疾病的導靶作用幾乎沒什麼方向性。然而逐漸地，現代藥物是通過準確導向分子水平的特定疾病狀態發現的，藥物可以是基因、基因的蛋白產物（如重組胰島素或促紅細胞生成素）或設計制造及修飾的小分子以調節特殊的疾病過程。無論新藥開發過程如何，所有的藥物都經過一個漫長的臨床前和臨床檢驗；第一個臨床前檢驗是測試藥物在動物模型中的存活力和藥用（通常是鼠或更高級的靈長類動物）；第二次檢驗測試在人體中的安全性、藥效和劑量。這些檢驗通過后（通常需要3-5年，花費5千萬到2億美金）最終要向本國政府授權機構提出申請（在美國是食品藥物管理委員會）。只有當該機構檢驗了藥物測試的數據并認為它具有良好的治療效果而沒有嚴重的副作用，這種藥物才被允許制造和銷售。

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.