People with addiction don’t start out thinking that they will become addicted to a substance. When they had their first few drinks or tokes thinking it would be a fun thing to do, it never occurred that a day would come when their life started to revolve around getting and taking drugs. That nothing but the drug would matter to them. It never occurred to them that there would come a time when their choice in the matter would be either severely limited or taken away entirely.
Through recent scientific advances, we know more about how drugs work in the brain than ever before. Brain imaging studies of drug-addicted individuals show changes in areas of the brain that are critical to judgment, decision making, learning and memory, and behavior control. Together, these changes can drive an abuser to seek out and take drugs compulsively despite adverse, even devastating consequences. When brain chemistry changes, the mind is tricked into thinking it NEEDS the drug to function. A physical addiction sets in and the person must have the drug.
When abuse becomes addiction
Almost all of us know someone who uses. How much we use ranges from abstinence (nothing at all) to experimental use to recreational use to chemically dependent use (addiction). At what point does someone who uses alcohol or drugs cross that line into addiction?
Addiction is a chronic disease characterized by drug seeking and use that is compulsive, or difficult to control, despite harmful consequences. The initial decision to take drugs is voluntary for most people, but repeated drug use can lead to brain changes that challenge an addicted person’s self-control and interfere with their ability to resist intense urges to take drugs. 
Here are a few indicators that addiction is setting in: 
Using drugs under dangerous conditions or taking risks while high
Getting into legal trouble.
Drug use causing problems for relationships
Building up a drug tolerance
Taking drugs to avoid or relieve withdrawal symptoms
Losing control over drug use
Life revolves around drug use
Abandoning activities you used to enjoy
Continuing to use drugs despite knowing it’s hurting you
From the perspective of a person with addiction, they need to take the drug to feel “normal” or to feel “good.” There is no choice.
There is no single factor that can predict whether you will become addicted to drugs. Risk for addiction is influenced by a combination of factors that include your individual biology and genetics, social environment, and age or stage of development. The more risk factors you have, the greater the chance that taking drugs can lead to addiction.
Although taking drugs at any age can lead to addiction, the earlier drug use begins, the more likely it will progress to a substance use disorder which poses a special challenge to adolescents. Because areas in the brain that govern decision-making, judgment, and self-control are still developing, teens may be especially prone to experimentation. 
Long-term use causes changes in other brain chemical systems and circuits as well. Glutamate is a neurotransmitter that influences the reward circuit and the ability to learn. When the optimal concentration of glutamate is altered by drug abuse, the brain attempts to compensate, which can impair cognitive function. 
Concerned about your drug use?
What role do drugs play in your life? Have you ever had a negative experience? Here are some tools to help you understand your drinking and drug use and suggestions for how to stay safe:
Understand your drinking: From Drink Aware in the U.K., here are some quizzes to help you understand your relationship with alcohol. They even have suggestions on what steps to take if you do want to make a change.
Check where I’m at with weed: “What’s with Weed” has a quiz you can take to see where you stand with marijuana. There are also tips to reduce your harms and see what others teens are saying about weed.
Drug use quiz: From mindcheck.ca, this quiz can help you identify if there is a problem. You can also learn about depression, anxiety, and body image. There are a lot of quizzes and resources for you to check out here.
CRAFFT: Designed specifically for you, the CRAFFT test is self-administered. You can take the test and if you answer yes to more than two of the questions consider talking with a trusted adult about your substance use.
Do you think you, or someone you know, needs a little extra help? Then you’re not alone. Many people in your own community struggle with similar problems. Your community has a lot of resources available to you including group meeting, people to talk to, and more. Check out this site for the latest on how drug affect brain and body.
In reality, drug addiction is a complex disease, and quitting takes more than good intentions or a strong will. In fact, because drugs change the brain in ways that foster compulsive drug abuse, quitting is difficult, even for those who are ready to do so. Through scientific advances, we know more about how drugs work in the brain than ever, and we also know that drug addiction can be successfully treated to help people reduce and even stop using drugs.
For the substance user, a life free from all chemicals is a terrifying prospect. However, it is possible to attain full recovery if treated professionally. To successfully arrest addictions, a total multi-disciplinary treatment approach is necessary — one that involves medical, behavioral, and social sciences, as well as philosophical and spiritual wisdom.
Some people may quit using willpower alone, but unless the illness has been properly treated, they are more likely to start drinking/using again. Unless and until someone with addiction achieves personal integrity and attains inner security, they are powerless to live free from chemicals.
"Understanding Drug Use and Addiction." DrugFacts. National Institute on Drug Abuse (NIDA). Web.
"Drug Abuse and Addiction. Signs, Symptoms, and Help for Drug Problems and Substance Abuse.” HELPGUIDE.ORG. Web.
Wilcox, Stephen. "Understanding Addiction." National Council on Alcoholism and Drug Dependence. Web.
"Drugs, Brains, and Behavior: The Science of Addiction." PsycEXTRA Dataset (n.d.): National Institute on Drug Abuse. Web.
"Genetics and Epigenetics of Addiction." DrugFacts: Genetics and Epigenetics of Addiction | National Institute on Drug Abuse (NIDA). February 2016. Web.