In the United Kingdom, as with all mental disorders, where a sufferer could potentially be a danger to themselves or others, they can be detained under the Mental Health Act 1983 for the purposes of assessment, care and treatment. This is a last resort, and usually avoided if the patient has family or friends who can ensure care.

The United Kingdom DVLA (Driving & Vehicle Licensing Agency) states that dementia sufferers who specifically suffer with poor short term memory, disorientation, lack of insight or judgement are almost certainly not fit to drive—and in these instances, the DVLA must be informed so said license can be revoked. They do however acknowledge low-severity cases and early sufferers, and those drivers may be permitted to drive pending medical report.

Various types of brain injury, occurring as a single event, may cause irreversible but fixed cognitive impairment. Traumatic brain injury may cause generalized damage to the white matter of the brain (diffuse axonal injury), or more localized damage (as also may neurosurgery). A temporary reduction in the brain's supply of blood or oxygen may lead to hypoxic-ischemic injury. Strokes (ischemic stroke, or intracerebral, subarachnoid, subdural or extradural hemorrhage) or infections (meningitis and/or encephalitis) affecting the brain, prolonged epileptic seizures and acute hydrocephalus may also have long-term effects on cognition. Excessive alcohol use may cause alcohol dementia, Wernicke's encephalopathy and/or Korsakoff's psychosis, and certain other recreational drugs may cause substance-induced persisting dementia; once overuse ceases, the cognitive impairment is persistent but not progressive.

Dementia which begins gradually and worsens progressively over several years is usually caused by neurodegenerative disease, that is, by conditions affecting only or primarily the neurons of the brain and causing gradual but irreversible loss of function of these cells. Less commonly, a non-degenerative condition may have secondary effects on brain cells, which may or may not be reversible if the condition is treated.

The causes of dementia depend on the age at which symptoms begin. In the elderly population (usually defined in this context as over 65 years of age), a large majority of cases of dementia are caused by Alzheimer's disease, vascular dementia or both. Dementia with Lewy bodies is another fairly common cause, which again may occur alongside either or both of the other causes. Hypothyroidism sometimes causes slowly progressive cognitive impairment as the main symptom, and this may be fully reversible with treatment. Normal pressure hydrocephalus, though relatively rare, is important to recognize since treatment may prevent progression and improve other symptoms of the condition. However, significant cognitive improvement is unusual.

Dementia is much less common under 65 years of age. Alzheimer's disease is still the most frequent cause, but inherited forms of the disease account for a higher proportion of cases in this age group. Frontotemporal lobar degeneration and Huntington's disease account for most of the remaining cases. Vascular dementia also occurs, but this in turn may be due to underlying conditions (including antiphospholipid syndrome, CADASIL, MELAS, homocystinuria, moyamoya and Binswanger's disease). People who receive frequent head trauma, such as boxers or some martial artists, are at risk of dementia pugilistica.

In young adults (up to 40 years of age) who were previously of normal intelligence, it is very rare to develop dementia without other features of neurological disease, or without features of disease elsewhere in the body. Most cases of progressive cognitive disturbance in this age group are caused by psychiatric illness, alcohol or other drugs, or metabolic disturbance. However, certain genetic disorders can cause true neurodegenerative dementia at this age. These include familial Alzheimer's disease, metachromatic leukodystrophy, SCA17 (dominant inheritance); adrenoleukodystrophy (X-linked); Gaucher's disease type 3, Niemann-Pick disease type C, pantothenate kinase-associated neurodegeneration, Tay-Sachs disease and Wilson's disease (all recessive). Wilson's disease is particularly important since cognition can improve with treatment.

At all ages, a substantial proportion of patients who complain of memory difficulty or other cognitive symptoms are suffering from depression rather than a neurodegenerative disease. Vitamin deficiencies and chronic infections may also occur at any age; they usually cause other symptoms before dementia occurs, but occasionally mimic degenerative dementia. These include deficiencies of vitamin B12, folate or niacin, and infective causes including cryptococcal meningitis, HIV, Lyme disease, progressive multifocal leukoencephalopathy, subacute sclerosing panencephalitis, syphilis and Whipple's disease.

Creutzfeldt-Jakob disease typically causes a dementia which worsens over weeks to months, being caused by prions. The common causes of slowly progressive dementia also sometimes present with rapid progression: Alzheimer's disease, dementia with Lewy bodies, frontotemporal lobar degeneration (including corticobasal degeneration and progressive supranuclear palsy).