Organic mental disorders involve a disruption of psychological functioning due to dysfunction of the brain. Primary degenerative dementia of the Alzheimer type refers to the progressive and deteriorating disorder in which cognitive abilities such as memory, thinking, and personality functioning are severely disrupted. In the early stages, the individual experiences memory impairment and social withdrawal. Eventually, personality changes occur and the individuals cognitive functions deteriorate to the point that he or she is incapable of caring for him/herself. This disorder is much more common in people older than 65 years (senile onset), but does not occur in younger individuals (presenile onset). Approximately 3 percent of the population over 65 have primary degenerative dementia of the Alzheimer type.

Organic mental disorders can also be caused by disease or by an accident. For example, brain tumors, exposure to lead, mercury, or other toxic chemicals, certain nervous disease, or advanced syphilis all can produce psychotic symptoms. The severity of the psychosis, however, is a function not just of the physical damage, but also of the person's individual personality traits and previous psychological disruption.

People also develop psychotic symptoms because they have taken certain drugs. Such as psychoactive substance induced organic mental disorders are a growing problem among the users of drugs such as LSD and "crack" cocaine, which interfere with the normal functioning of the nervous system. Other substances that can lead to organic mental disorders include: alcohol, amphetamines, caffeine, cannabis, cocaine, hallucinogens, inhalants, nicotine, opioids, phencyclidine, and sedatives.