Drugs can be bad for you in lots of ways. Even occasional or experimental drug use can be dangerous, since drugs can have unexpected adverse health effects even with one use; and drugs affect your ability to exert good judgment, making it more likely that you might engage in risky behaviors that can have serious consequences, such as driving while intoxicated. Prolonged drug use can cause all sorts of medical problems—like lung cancer, heart disease, liver disease, and addiction. When someone is addicted to drugs, they become the most important thing in that person's life, causing them major problems at school, home, and work.
Teens are in a period of tremendous upheaval regarding how their bodies and brains are changing. In fact, scientists only recently learned how much brain development goes on during the adolescent years and well into early adulthood. This means that the teen brain is wired somewhat differently from the adult brain and that exposure to drugs (or other important stimuli) during this phase can affect how the brain develops. We know that teen decision-making is often different from that of adults, and can involve more risk-taking. Some of this is good, helping teens learn who they are and what they want to be, but some risks can have serious negative consequences as well—a factor that's less impressive to the teen brain. What we do know is that early drug use is associated with later drug problems. Whether this is because of changes in the brain that are especially prominent during adolescence or other factors, such as co-occurring depression or anxiety, or exposure to trauma or stress, is not yet clear.
That's not a simple question to answer. For example, most people who use marijuana do not go on to use other illicit drugs, but most people who use other drugs have previously used marijuana. For those people, did the marijuana use lead to the use of other drugs? Possibly, and for a number of different reasons. Being exposed to peers who use drugs, having greater access to drugs, having greater problems in the first place that led to the initial drug use, or persistent effects of the drug on the brain could all make other drug use more likely. The latter, in fact, is supported by some research done in animals.
We know from scientific research that the earlier you start using drugs, the more likely you are to become addicted and suffer serious social and medical consequences. The reasons for this are complex—first, drugs affect the brain, and the brain is still developing until early adulthood. So, it's possible for drugs to alter the normal developmental pattern—research is still ongoing to help us figure this out. Second, people who use drugs when they are very young often have other problems that led to their drug use in the first place. For example, they may have difficult family situations or problems with depression or anxiety, and they use drugs to help them cope. Unfortunately, drug use just makes things worse in the long run, and doesn't address the problems that led to its use. Third, using drugs can interfere with success in school, in sports, and in relationships with friends and family, further creating problems down the road.
So the bottom line is that early drug use can lead to later drug use and other problems, and the best advice is to never start. But if you have started using already, you should know that the earlier you stop, the more likely you will be to avoid addiction and the other harmful consequences associated with it.
For more information, see https://www.drugabuse.gov/publications/drugs-brains-behavior-science-addiction.
Drug use and addiction and other mental disorders are intimately related, so this question is like a coin that can be looked at from two different sides. On one hand, we have known for a long time that people who use drugs, in general, have higher rates of many mental illnesses. In fact, there is evidence that drug use early in life may increase the risk of psychiatric disorders or accelerate their course. On the other hand, it is equally true that persons with mental illnesses are more likely to use drugs than other people. For example, children and adolescents with conduct disorders, attention deficit hyperactivity disorder (ADHD), and learning disabilities are at higher risk of using drugs than other youths, particularly if the mental illness is left untreated.
The reasons why addiction and other mental disorders are so closely linked and often coincide (or co-occur) in the same person are not fully understood. But scientists believe there may be at least three different reasons for this:
- Drug use may cause one or more symptoms of another mental illness, through long-term negative changes in brain structure and function. For example, repeated cocaine use can cause panic attacks, which can then persist even when a person stops using cocaine.
- Mental illnesses may lead to drug use, if, for example a mentally ill person begins to use a drug in an attempt to self-medicate (i.e., to reduce or manage the symptoms of the disorder or those caused by a medication prescribed for the disorder). The high rates of tobacco addiction in people with schizophrenia may be linked to reducing the cognitive disturbances of schizophrenia or countering medication side effects.
- Drug use and other mental disorders may be both caused by common factors, such as underlying brain deficits or early exposure to stress or trauma. NIDA-supported investigators are using neuroimaging, genotyping, statistical modeling, and other tools to better understand the interplay of risk factors in the development of such disorders.
For further information see: http://www.drugabuse.gov/news-events/nida-notes/2007/02/addiction-co-occurring-mental-disorders
People who are depressed sometimes turn to using illicit drugs to feel better, but that would be a temporary solution at best. More typically it is asking for more serious trouble. Illicit drugs can make you feel high for a little while soon after you take them, but when they leave your body you won't feel any better—and you may feel worse—than before you took them. With continued drug use, the period of feeling good grows shorter and the subsequent bad feelings get worse. If you or someone you know feels depressed, talk to a doctor. There are many treatments for depression. There are medications that have been tested and approved by the FDA, and there are psychosocial or behavioral treatments that can help improve feelings of depression that do not involve taking medications.