Continued use of substances that act on the nervous system is responsible for physical dependence as your body adjusts to them and the brain undergoes a change in structure and performance, so that it can only function normally in the presence of such chemicals, without forgetting that increasingly requires greater consumption for pleasurable effects.
It is important to note that each substance acts in a particular stimulant in the brain and not all produce physical dependence or withdrawal. To better explain, we can resort to a general classification of these chemicals into three groups:
Depressants. Small doses produce euphoria, but when relaxed high brain activity, causing drowsiness and lethargy, even death by paralyzing the nerve activity that controls the respiratory system. Many of these chemicals are used for medical purposes, such as sedatives (narcotics) or sleep aids (hypnotics). The best known in this classification are: alcohol, benzodiazepines, sedatives drugs, solvents and adhesives, the first three produce physical and psychological dependence, the latter two only psychological, as has been proven to date.
Stimulants. Increase the activity of central nervous system. Tend to generate improvement in mood, feeling of happiness, apparent increase in mental and physical abilities, lack of sleep and appetite, and increased activity in general. This group includes amphetamine, methamphetamine (speed), methylenedioxymethamphetamine (ecstasy or MDMA), phencyclidine (PCP or “angel dust”), cocaine, nicotine, and caffeine, all these substances have physical and psychological dependence to varying degrees, plus syndrome abstinence.
Hallucinogens or psychedelics. The impression of perceiving images or sounds that ordinarily can not be received through the senses, and are used by some cultures in rituals. Is leading to increased activity of certain brain centers, dilated pupils, cold arms and legs, and tingling sensations in the stomach also come to generate very pleasant or terrifying experiences.
In this category are: lysergic acid diethylamide (LSD), marijuana, mescaline (peyote) and psilocybin, all produce psychological dependence, while the physical type and the withdrawal have not been proven.
Years ago he used to talk about two groups of drugs, “hard” to create physical and psychological dependence, such as alcohol, opiates, amphetamines and barbiturates, and “soft” that only believed to be responsible for psychological dependence, as marijuana, cocaine, LSD and snuff. This classification has been widely criticized, both because it creates the idea that substances are “good” and “bad” as it has been shown that some of the chemicals in the second group do create tolerance and withdrawal.