Addictive substances are chemicals or materials which can create physical and/or psychological dependencies. The U.S. government classifies these substances according to their potential for addiction and medical uses.
Certain of them are strictly illegal to use, but others can be used under medical supervision. Some addictive substances that can be legally sold over-the-counter are caffeine, nicotine, alcohol, inhalants, cough medications, and certain psychotropic herbs. Common ones with medical uses that are illegal to use without a doctor's prescription are cocaine, narcotic painkillers such as hydrocodone and oxycodone, barbiturates, benzodiazepines, and methamphetamine. Heroin and certain hallucinogens have no medical uses and are illegal to use at all. Marijuana is currently classified that way by the federal government, even though certain states have made marijuana legal to use for medical purposes.
A person using an "addictive substance" can develop what doctors call "substance abuse disorder" or "substance dependency." The difference between the two disorders is that those "dependent" on substances go through physical withdrawal syndromes when they quit. Symptoms can be so severe that some people need medical supervision and medications to get through them. Or withdrawal can be simply uncomfortable, such as getting headaches when you quit drinking coffee. Avoiding the unpleasantness of withdrawal symptoms is one major reason substance abusers keep using their drug, cigarettes, etc.
Even if they do not have to undergo physical withdrawal syndromes, people can find it hard to overcome psychological dependencies on substances. For example, ketamine and LSD are drugs that do not produce physical withdrawal symptoms when you quit using them. However, people can become so involved in a lifestyle that involves clubbing and using hallucinogens or so overly fascinated by altered states of consciousness that it becomes difficult for them to quit without professional help.
Recent research indicates that certain foods, especially sweets and high-fat, salty foods, could be addictive substances that create dependencies.
Signs of addiction or substance dependency disorder is an inability to quit even when you have the desire to, failing at quitting on your own, having to keep the substance nearby, feeling out of control about your substance use, using the substance as a coping mechanism, spending too much time getting and using the substance, and developing legal problems or difficulties in relationships because of substance use.