Drug addiction, also called substance use disorder, is a dependence on a legal or illegal drug or medication.Keep in mind that alcohol and nicotine are legal substances, but are also considered drugs.When you're addicted, you're not able to control your drug use and you may continue using the drug despite the harm it causes. Drug addiction can cause an intense craving for the drug.You may want to quit, but most people find they can't do it on their own. Drug addiction can cause serious, long-term consequences, including problems with physical and mental health, relationships, employment, and the law.You may need help from your doctor, family, friends, support groups or an organized treatment program to overcome your drug addiction and stay drug-free. 1  Symptoms.eep in mind that alcohol and nicotine are legal substances, but are also considered drugs. When you're addicted, you're not able to control your drug use and you may continue using the drug despite the harm it causes. Drug addiction can cause an intense craving for the drug.