A drug is a substance used as medicine in the treatment of disease, or a moodaltering substance, especially one that is addictive. Drugs are chemical agents that affect the systems of the body. In general they are taken to treat or prevent disease, but certain drugs, such as the opium narcotics, amphetamines, barbiturates and hallucinogens, are taken for their psychological effects and are drugs of addiction or abuse.
Many drugs are the same as or similar to chemicals occurring naturally in the body and are used either to replace the natural substance (for instance, thyroid hormone) when deficient, or to induce effects that occur when these is an abnormal concentration of the substance, such as with steroids or oral contraceptives.
Other agents are known to interfere with a specific mechanism or to antagonize a normal process. Many other drugs are obtained from other biological systems; fungi, bacteria (antibiotics), plants (e.g. digitalis) and several others are chemical modifications of natural products. In addition, there are a number of entirely synthetic drugs (e.g. barbiturates), some of which are based on active parts of naturally occurring drugs.