What are the drugs involved?
The three drugs which look likely to be available to patients with "mild" Alzheimer's are Aricept (donepezil), Reminyl (galantamine) and Exelon (rivastigmine).
In addition, NICE says a fourth drug, Ebixa, should be prescribed to patients with more advanced Alzheimer's.
How do they work?
One theory of the origins of Alzheimer's disease in some patients is damage to the "cholinergic" system in the brain, which leads to a shortage of a neurotransmitter called acetylcholine which has been linked to memory function in Alzheimer's.
All of the three drugs work by inhibiting an enzyme which breaks down acetylcholine - hopefully boosting levels of the brain chemical.
What benefits have been seen?
The drugs do not cure Alzheimer's. There is no proof that the drugs can alter the course of the disease or extend life. However, there is evidence that they can lessen symptoms such as memory loss and confusion in patients with "mild" Alzheimer's. This in turn can improve quality of life, and allow patients to live more independently for longer. One analysis conducted by NICE suggests that they could delay the need for someone to go into full-time care by over six weeks.
However, they do not work for everyone. It is estimated that between 40% and 70% of patients achieve some kind of benefit, and side effects can include dizziness, insomnia and nausea.
Why have the guideline changes been proposed?
NICE says that the publication of further clinical studies into the drugs in the five years since they were last appraised show more clearly the benefits to patients with "mild" Alzheimer's.
Campaigners had argued that the "economic model" used to calculate whether the drugs offered economic "value for money", did not accurately reflect the hidden economic costs of extra care outside hospital.
NICE says this time around it considered new evidence on the possible cost savings of treating early Alzheimer's patients.
How much more will it cost the NHS?
This is not entirely clear yet. The cost of the drugs themselves is roughly £1,000 per year, per patient, but the benefits of the treatment do not continue forever, and the exact number of patients who will benefit has yet to be determined. NICE calculates that more than 380,000 have Alzheimer's, approximately half of these being in the "mild to moderate" group.
The Alzheimer's Society believes the number is higher, but with an estimated 62,000 people developing Alzheimer's each year, it is likely that hundreds of thousands of people could potentially be eligible over the next few years.
However, NICE says it is confident that the extra cost of caring for these patients outweighs the cost of the drugs themselves.