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Drug, substance that affects the function of living cells, used in medicine to diagnose, cure, prevent the occurrence of diseases and disorders, and prolong the life of patients with incurable conditions.

Since 1900 the availability of new and more effective drugs such as antibiotics, which fight bacterial infections, and vaccines, which prevent diseases caused by bacteria and viruses, has increased the average American’s life span from about 60 years to about 75 years. Drugs have vastly improved the quality of life. Today, drugs have contributed to the eradication of once widespread and sometimes fatal diseases such as poliomyelitis and smallpox.


Drugs can be classified in many ways: by the way they are dispensed——over the counter or by prescription; by the substance from which they are derived—plant, mineral, or animal; by the form they take—capsule, liquid, or gas; and by the way they are administered—by mouth, injection, inhalation, or direct application to the skin (absorption). Drugs are also classified by their names. All drugs have three names: a chemical name, which describes the exact structure of the drug; a generic or proprietary name, which is the official medical name assigned by the United States Adopted Name Council (a group composed of pharmacists and other scientists); and a brand or trade name given by the particular manufacturer that sells the drug. If a company holds the patent on a drug—that is, if the company has the exclusive right to make and sell a drug, then the drug is available under one brand name only. After the patent expires, typically after 17 years in the United States, other companies can also manufacture the drug and market it under the generic name, or give it a new brand name.

Another way to categorize drugs is by the way they act against diseases or disorders: chemotherapeutic drugs attack specific organisms that cause a disease without harming the host, while pharmocodynamic drugs alter the function of bodily systems by stimulating or depressing normal cell activity in a given system. The most common way to categorize a drug is by its effect on a particular area of the body or a particular condition.

A Endocrine Drugs

Endocrine drugs correct the overproduction or underproduction of the body’s natural hormones. For example, insulin is a hormone used to treat diabetes. The female sex hormones estrogen and progesterone are used in birth control pills. Estrogen may be given as a replacement therapy to relieve uncomfortable symptoms associated with menopause including sweating, hot flashes, and mood swings. Estrogen replacement therapy may also delay some long-term consequences of menopause including osteoporosis and atherosclerosis.

B Anti-infective Drugs

Anti-infective drugs are classified as antibacterials, antivirals, or antifungals depending on the type of microorganism they combat. Anti-infective drugs interfere selectively with the functioning of a microorganism while leaving the human host unharmed.

Antibacterial drugs, or antibiotics—sulfa drugs, penicillins, cephalosporins, and many others—either kill bacteria directly or prevent them from multiplying so that the body’s immune system can destroy invading bacteria. Antibacterial drugs act by interfering with some specific characteristics of bacteria. For example, they may destroy bacterial cell walls or interfere with the synthesis of bacterial proteins or deoxyribonucleic acid (DNA)—the chemical that carries the genetic material of an organism. Antibiotics often cure an infection completely. However, bacteria can spontaneously mutate, producing strains that are resistant to existing antibiotics.

Antiviral drugs interfere with the life cycle of a virus by preventing its penetration into a host cell or by blocking the synthesis of new viruses. Antiviral drugs may cure, but often only suppress, viral infections; and flare-ups of an infection can occur after symptom-free periods. With some viruses, such as human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS), antiviral drugs can only prolong life, not cure the disease.

Vaccines are used as antiviral drugs against diseases such like mumps, measles, smallpox, poliomyelitis, and influenza. Vaccines are made from either live, weakened viruses or killed viruses, both of which are designed to stimulate the immune system to produce antibodies, proteins that attack foreign substances. These antibodies protect the body from future infections by viruses of the same type (see Immunization).

Antifungal drugs selectively destroy fungal cells by altering cell walls. The cells’ contents leak out and the cells die. Antifungal drugs can cure, or may only suppress, a fungal infection.

C Cardiovascular Drugs

Cardiovascular drugs affect the heart and blood vessels and are divided into categories according to function. Antihypertensive drugs reduce blood pressure by dilating blood vessels and reducing the amount of blood pumped by the heart into the vascular system. Antiarrhythmic drugs normalize irregular heartbeats and prevent cardiac malfunction and arrest.

D Drugs that Affect the Blood

Antianemic drugs, such as certain vitamins or iron, enhance the formation of red blood cells. Anticoagulants like heparin reduce blood-clot formation and ensure free blood flow through major organs in the body. Thrombolytic drugs dissolve blood clots, which can block
blood vessels and deprive the heart or brain of blood and oxygen, possibly leading to heart attack or stroke.

E Central Nervous System Drugs

Central nervous system drugs—that is, drugs that affect the spinal cord and the brain—are used to treat several neurological (nervous system) and psychiatric problems. For instance, antiepileptic drugs reduce the activity of overexcited brain areas and reduce or eliminate seizures.

Antipsychotic drugs are used to regulate certain brain chemicals called neurotransmitters, which do not function properly in people with psychoses, major mental disorders often characterized by extreme behaviors and hallucinations, such as in schizophrenia. Antipsychotic drugs can often significantly alleviate hallucinations and other abnormal behaviors.

Antidepressant drugs reduce mental depression. Antimanic drugs reduce excessive mood swings in people with manic-depressive illness, which is characterized by behavioral fluctuations between highs of extreme excitement and activity and lows of lethargy and depression. Both types of drugs act by normalize chemical activity in the emotional centers of the brain. Antianxiety drugs, also referred to as tranquilizers, treat anxiety by decreasing the activity in the anxiety centers of the brain.

Sedative-hypnotic drugs are used both as sedatives to reduce anxiety and as hypnotics to induce sleep. Sedative-hypnotic drugs act by reducing brain-cell activity. Stimulatory drugs, on the other hand, increase neuronal (nerve cell) activity and reduce fatigue and appetite.

Analgesic drugs reduce pain and are generally categorized as narcotics and non-narcotics. Narcotic analgesics, also known as opioids, include opium and the natural opium derivatives codeine and morphine; synthetic derivatives of morphine such as heroin; and synthetic drugs such as meperidine and propoxyphene hydrochloride. Narcotics relieve pain by acting on specific structures, called receptors, located on the nerve cells of the spinal cord or brain. Non-narcotic analgesics such as aspirin, acetaminophen, and ibuprofen reduce pain by inhibiting the formation of nerve impulses at the site of pain. Some of these drugs can also reduce fever and inflammation.

General anesthetics, used for surgery or painful procedures, depress brain activity, causing a loss of sensation throughout the body and unconsciousness. Local anesthetics are directly applied to or injected in a specific area of the body, causing a loss of sensation without unconsciousness; they prevent nerves from transmitting impulses signaling pain (see Anesthesia).

F Anticancer Drugs

Anticancer drugs eliminate some cancers or reduce rapid growth and spread. These drugs do not affect all cancers but are specific for cancers in certain tissues or organs such as the bladder, brain, liver, or bones. Anticancer drugs interfere with specific cancer-cell components. For example, alkylating agents are cytotoxic (cell-poisoning) drugs that alter the DNA of cancer cells. Vinca alkaloids, chemicals produced by the periwinkle plant, prevent cancer cell division.

G Other Drugs

Many other categories of drugs also exist, such as anti-inflammatory, antiallergic, antiParkinson (see Parkinson Disease), antiworm (see Anthelmintic Drugs), diuretic, gastrointestinal, pulmonary, and muscle-relaxant drugs. Often a drug in one category can also be used for problems in other categories. For example, lidocaine can be used as a local anesthetic or as a cardiac drug.


The effect of a drug on the body depends on a number of processes that the drug undergoes as it moves through the body. All these processes together are known as pharmacokinetics (literally, “motion of the drug”). First in these processes is the administration of the drug after which it must be absorbed into the bloodstream. From the bloodstream, the drug is distributed throughout the body to various tissues and organs. As the drug is metabolized, or broken down and used by the body, it goes through chemical changes that produce metabolites, or altered forms of the drug, most of which have no effect on the body. Finally, the drug and its metabolites are eliminated from the body.

A Administration

Depending on the drug and its desired effect, there are a variety of administration methods. Most drugs are administered orally—that is, through the mouth. Only drugs that will not be destroyed by the digestive processes of the stomach or intestines can be given orally. Drugs can also be administered by injection into a vein (intravenously), which assures quick distribution through the bloodstream and a rapid effect; under the skin (subcutaneously) into the tissues, which results in localized action at a particular site as with local anesthetics; or into a muscle (intramuscularly), which enables rapid absorption through the many blood vessels found in muscles. An intramuscular injection may also be given as a depot preparation, in which the drug is combined with other substances so that it is slowly released into the blood.

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