Author Terry Pratchett has criticised a decision to limit the drug Aricept through the NHS to people in the later stages of Alzheimer's disease.
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Terry Pratchett on Panorama
He told the BBC's Panorama programme the National Institute for Health and Clinical Excellence's ruling "feels like an insult" and needed a rethink.
NICE decided the drug was not cost-effective in the early stages.
Pratchett, who has sold 55 million books worldwide, has PCA, a rare early-onset form of the disease.
He was diagnosed with PCA, which affects the back of the brain and therefore vision and motor skills, in December 2007.
Speaking to Panorama for its programme The NHS Postcode Lottery - It Could be You, he said his doctor told him about Aricept straight after he was diagnosed, but she initially said she could not prescribe it for him.
"I didn't have a specialist at that time and so I was a bit in limbo," he said. "I think she took pity and sort of did it."
He now pays for his Aricept, which is prescribed to him by a specialist in Bath, but is highly critical of the Byzantine route he had to take to get the drugs he says he needs.
"It's probably easier to get drugs off Fat Charlie round the back of the bus station than it is to get medicines - but there we are," he told Panorama.
The head of NICE, Andrew Dillon, told Panorama: "The evidence is absolutely clear."
He said the extent of the benefits and the numbers of patients involved meant the NHS should not give the drug to patients before they moved into the moderate stage of the disease.
NICE guidance in 2001 recommended the drug - which can make it easier to carry out everyday tasks - should be used as standard.
But revised advice, published in November 2006, stated that Aricept should be prescribed only to people with moderate-stage disease.
The drug Aricept costs about £2.50 per day
NICE said that Aricept, which costs about £2.50 a day, did not make enough of a difference to recommend it for all patients and therefore was not good value for money.
Pratchett acknowledges that through his success as an author - his Discworld series has sold 55 million books worldwide - he is easily able to afford to Aricept.
But he is concerned for other people with Alzheimer's who lack his means.
"I feel particularly angry on behalf of early-onset patients because it feels like an insult and the younger you are the more insulting it is," he says.
"A lot of people with early onset have got dependants both younger and older and they're trying to hold down a job.
"It is a really nasty disease and I cannot imagine cancer patients being denied a drug like this in similar circumstances," Pratchett added.
He says that he has noticed a huge change in his condition since he started taking Aricept.
"If I'm not on the drug I'd find certain things difficult, doing up buttons so you get a line of buttons in the shirt done up right can be a problem," he said.
Alzheimer's mainly affects older people
"In theory I should be that much worse, but my wife has said that I'm better now than I was in the autumn."
But Pratchett says that the advantages of taking the drug are wider than that.
"Alzheimer's scares people and at four o'clock in the morning it scares me, and Aricept is well worth having for the relief that it brings."
He said that the decision not to give it to early stage sufferers, but to provide free treatment for obesity and sexual impotence, indicated the NHS "priorities are not right".
And the author also told Panorama that he thought it was possible that NICE's decision was in part shaped by the fact that Alzheimer's was a disease predominantly affecting older people, who he said were thought of as "a softer target".
Panorama: The Postcode Lottery: It Could Be You will be on BBC One at 8.30pm on Monday 18 August.