指能夠影響生物功能的化學物質，可供治療、診斷和預防疾病用。常用藥包括有抗生素、興奮劑、安定劑、抗抑鬱藥、鎮痛藥、麻醉性鎮痛藥、荷爾蒙和各種特定用途藥，如輕瀉藥、強心劑、抗凝血藥、利尿劑和抗組織胺藥。疫苗有時也被視為一種藥物。醫學藥物的作用有︰可保護以對抗入侵的有機體（或殺死它們、或中止其複製，或阻斷其對宿主的影響）；補充體內缺少的物質或將之排出體外；干擾一項不正常的程式。藥物的使用法有口服、肛門塞劑、皮下注射、肌肉注射、靜脈注射和吸入等。已知最古老的藥典是西元前1700年古巴比倫尼亞的一塊石碑；1928年抗生素的發明象徵了現代藥物時代的來臨。今日，多數藥物是由化學法合成，也有許多重要藥物（如許多抗生素和疫苗）是由植物、動物、礦物和微生物中萃取純化而得。使用藥物不止要其反應也要安全；副作用的程度可從輕微到危險（參閱medicinal poisoning）。許多違禁藥物也可以用於醫學上（參閱cocaine、heroin和drug addiction）。亦請參閱drug resistance、pharmacology、pharmacy。
drug
Any chemical agent that affects the function of living things. Some, including antibiotics, stimulants, tranquilizers, antidepressants, analgesics, narcotics, and hormones, have generalized effects. Others, including laxatives, heart stimulants, anticoagulants, diuretics, and antihistamines, act on specific systems. Vaccines are sometimes considered drugs. Medicinal drugs may protect against attacking organisms (by killing them, stopping them from reproducing, or blocking their effects on the host), substitute for a missing or defective substance in the body, or interrupt an abnormal process. A drug must bind with receptors in or on cells and cannot work if the receptors are absent or its configuration does not fit theirs. Drugs may be given by mouth, by injection, by inhalation, rectally, or through the skin. The oldest existing catalogue of drugs is a stone tablet from ancient Babylonia (c. 1700 BC); the modern drug era began when antibiotics were discovered in 1928. Synthetic versions of natural drugs led to design of drugs based on chemical structure. Drugs must be not only effective but safe; side effects can range from minor to dangerous (see medicinal poisoning). Many illegal drugs also have medical uses (see cocaine, heroin, drug addiction). See also drug resistance, pharmacology, pharmacy.
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