The word dementia is used to describe a group of symptoms – including memory loss, confusion, and communication difficulties. Alzheimer's disease is the most common cause of dementia.

What is Alzheimer’s disease?

As we age, our brains naturally shrink a little and our thought processes slow down. However, in Alzheimer’s disease, changes occur in the brain that are different to the changes seen in normal ageing.

In Alzheimer’s disease, two proteins, called amyloid and tau, build up.

Amyloid builds up in the space between different cells in the brain, and tau is present inside nerve cells themselves, as seen in the diagram below.

Nerve cell-healthy & damaged-Together

Although we don’t yet have a complete understanding of what triggers these proteins to build up, research suggests that these proteins damage more and more brain cells over time.

This damage affects how our brain works and leads to dementia symptoms. You can find out more about what we know here.

Some people can have more than one cause of their dementia. For example, someone might have Alzheimer’s as well as vascular dementia or dementia with Lewy bodies. This is often called ‘mixed dementia’.

Alzheimer’s is not a normal part of ageing, but the chance of developing the disease increases as we get older. Most people who have Alzheimer’s are over the age of 65, and this is called ‘late onset’ Alzheimer’s.

Sometimes, Alzheimer’s can affect younger people too. It’s thought that at least three in every 100 people with Alzheimer’s in the UK are under 65. These uncommon cases of the disease are called ‘young onset’ Alzheimer’s disease.

People with young onset Alzheimer’s are more likely to have inherited forms of the disease than those with late onset Alzheimer’s.

Have you or a loved one recently been diagnosed with Alzheimer's disease?

If you have been diagnosed with Alzheimer's disease in the last two years and would like to share your experience to help shape our work please get in touch via 

What is Alzheimer's disease?

Find out more about the symptoms and causes of Alzheimer's disease, and the treatments currently available.

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This information was updated in May 2024 and is due to be reviewed in May 2026. It was written by Alzheimer’s Research UK’s Information Services team with input from lay and expert reviewers. Please get in touch if you’d like a version with references or in a different format.

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