A drug is any chemical substance that changes the way the body functions. Mood-altering or psychoactive drugs affect the way a person thinks, feels, and acts.

Examples: Antihistamines reduce the symptoms of allergies. Cough medicines decrease coughing and can make a person feel more relaxed. Consuming alcohol can lead to intoxication.

Harmful involvement
Harmful involvement is the use of a drug to the extent that it interferes with everyday life.
Example of harmful involvement: The adult who has a drink in the evening to wind down after work may not be harmfully involved with alcohol. But if the person is consuming more than two drinks each evening, and more than 14 a week (9 for women), then he or she may be harmfully involved.

Tolerance occurs when the body adjusts to a drug to the point that increased amounts are required to achieve the initial effects. An adult who finds one drink relaxing may, after a while, discover that it takes two or three drinks to achieve the same effect. This is developing a tolerance toward alcohol.
Examples of tolerance: A person often doesn’t realize that he or she is becoming tolerant to something. Freshly baked bread or cookies smell wonderful when you first enter the kitchen, but the smell quickly wears off when you get used to it. Swimming pools often feel very cold when you first jump in, but in a short time the water feels comfortable.

Physical dependence
Physical dependence occurs when the body becomes so accustomed to a particular drug that it can function normally only if the drug is present. Without the drug, the user may experience a variety of symptoms ranging from mild discomfort to convulsions, depending on the drug. These symptoms, some of which can be fatal, are collectively referred to as “withdrawal.” Not all drugs produce physical dependence, but they may still be abused because of their perceived effects, or psychological dependence. Physical dependence is one of the factors contributing to the continued use of drugs.
Example of physical dependency:If a smoker runs out of cigarettes, he or she may become anxious, agitated, restless, or depressed and have sleep disturbances and decreased blood pressure and heart rate. In heavy smokers these symptoms may develop within hours of the last cigarette.

Withdrawal describes the effects when a person stops taking a drug or reduces the amount of the drug. Usually the effects of withdrawal are the opposite of the effects experienced when the drug is taken.
Example of withdrawal: The person who has a cup of coffee every morning to wake up may feel drowsy or have a headache on a morning when he or she misses a cup of coffee.

Psychological dependence
Psychological dependence exists when a drug is so central to a person’s thoughts, emotions, and activities that it is extremely difficult to stop using it or even stop thinking about it. Like physical dependence, psychological dependence is the result of ongoing drug use and a cause of continued use.
Example of psychological dependency: People who have quit smoking can occasionally have strong cravings for a cigarette. This can occur years after the person has had his or her last cigarette.

While there is no universally accepted definition of addiction, it is commonly understood to refer to repeated use of a psychoactive substance or substances to the extent that the user is periodically or regularly intoxicated, shows a compulsion to take the preferred substance(s), has great difficulty in voluntarily stopping or modifying use, and attempts to obtain the substance(s) by almost any means.
Example of addiction: A person who is addicted to an opiate such as heroin, and unable to secure any other supply, breaks into a pharmacy to obtain opiate-based medicines such as Dilaudid or morphine.