Brain fog is a common problem occurring in people of any age, and can be dramatically worse in people with fibromyalgia. This should be distinguished from the cognitive dysfunction caused by Alzheimer's and dementia. Yet it frequently isn't. How to tell the difference? Brain fog is usually evidenced by having difficulty finding a word or a substitution of a word, and problems with short-term memory. But reasoning skills remain intact.
Simply put, with brain fog you keep forgetting where you left the keys. While with Alzheimer's you forget how to use the keys.