(From https://www.drugabuse.gov/frequently-asked-questions#physical)

What is drug addiction?

Drug addiction is a complex, and often chronic, brain disease. It is characterized by drug craving, seeking, and use that can persist even in the face of devastating life consequences. Addiction results largely from brain changes that stem from prolonged drug use—changes that involve multiple brain circuits, including those responsible for governing self-control and other behaviors. Drug addiction is treatable, often with medications (for some addictions) combined with behavioral therapies. However, relapse is common and can happen even after long periods of abstinence, underscoring the need for long-term support and care. Relapse does not signify treatment failure, but rather should prompt treatment re-engagement or modification.

How quickly can I become addicted to a drug?

There is no easy answer to this common question. If and how quickly you become addicted to a drug depends on many factors, including your biology (your genes, for example), age, gender, environment, and interactions among these factors. Vast differences characterize individual sensitivity to various drugs and to addiction vulnerability. While one person may use a drug one or many times and suffer no ill effects, another person may overdose with first use, or become addicted after a few uses. There is no way of knowing in advance how quickly you will become addicted—but there are some clues, one important one being whether you have a family history of addiction.

How do I know if someone is addicted to drugs?

If a person is compulsively seeking and using a drug(s) despite negative consequences, such as loss of job, debt, family problems, or physical problems brought on by drug abuse, then he or she probably is addicted. And while people who are addicted may believe they can stop any time, most often they cannot, and will need professional help—first to determine if they in fact are addicted, and then to obtain drug abuse treatment. Support from friends and family can be critical in getting people into treatment and helping them to maintain abstinence following treatment.

What is withdrawal? How long does it last?

Withdrawal describes the various symptoms that occur after long-term use of a drug is reduced or stopped abruptly. Length of withdrawal and symptoms vary with the type of drug. For example, physical symptoms of heroin withdrawal may include restlessness, muscle and bone pain, insomnia, diarrhea, vomiting, and cold flashes. These physical symptoms may last for several days, but the general depression, or dysphoria (opposite of euphoria), that often accompanies heroin withdrawal may last for weeks. In many cases, withdrawal can be easily treated with medications to ease the symptoms, but treating withdrawal is not the same as treating addiction.