While Great Britain's National Institute for Health and Clinical Excellence (NICE) was limiting that country's Alzheimer's patients' ability to access treatment, the FDA was expanding access here in the United States.

Japan's fourth-largest pharmaceutical, Eisai (OTC BB: ESALY.PK), won the right to have its best-selling drug Aricept prescribed for moderate to severe symptoms of Alzheimer's. Previously, the drug -- which is co-marketed with Pfizer (NYSE:PFE) -- was limited to only treating mild to moderate forms of the debilitating illness. It is now the only approved drug that covers treatment for the entire spectrum of Alzheimer's.

Prior to the ruling, the only drug that had been approved for treating advanced symptoms of Alzheimer's was memantine, which is marketed by Forest Labs (NYSE:FRX) as Namenda. Eisai has the chance to steal a lot of business from Forest Labs, though the company downplayed the significance, noting that the two drugs are already often prescribed together. However, these are expensive treatments, and now doctors may prescribe Aricept alone, in the interest of saving money. Forest Labs' application to have Namenda prescribed for mild to moderate forms of Alzheimer's was declined last year.

The market for Alzheimer's treatment is unfortunately large and growing. Worldwide sales of all treatments are estimated to be around $3 billion, and the market is expected to grow to $5.5 billion by 2009, according to Millennium Research Group.

Eisai's sales of Aricept grew to $1.7 billion when it reported results back in May, an increase over the prior year, and accounted for 47% of the pharmaceutical's sales. Forest Labs reported Namenda sales of $155.6 million in the latest quarter, up 26% for the year before. It is one of the pharmaceutical's two best-selling drugs, the other being the SSRI anti-depressant Lexapro. Yet it also faces potential sales losses, as Lexapro's primary competition, Pfizer's Zoloft, has lost patent protection and cheaper generics will soon be coming to market.

There is no cure for Alzheimer's, only methods of slowing its progression. Aricept is an acetylcholinesterase inhibitor, which means that it blocks the breakdown of acetylcholine, a chemical messenger considered to be critical to memory, thought, and judgment. This is important, since patients with Alzheimer's disease have low levels of acetylcholine. Aricept increases the amount of acetylcholine in the brain, and improves nerve cells' response to it.

Other treatments for mild to moderate forms of Alzheimer's include Exelon, which is made by Novartis (NYSE:NVS), Ebixa by H. Lundbeck, and Razadyne by Johnson & Johnson (NYSE:JNJ) and Shire Pharmaceuticals (NASDAQ:SHPGY). They are only approved for mild to moderate forms of Alzheimer's.

While Aricept is a lucrative drug for Eisai, which has patent protection until 2010, generic drug companies are targeting it and challenging the patents in court. Teva Pharmaceuticals (NASDAQ:TEVA) is being sued by Eisai for patent infringement, and the two are also wrangling over Eisai's other patent-protected drug, Aciphex, which is used for treating heartburn. Because of the high costs associated with Aricept, it is expected that generics will make quick inroads on Eisai's market when the patents expire.

In the interim, Eisai can now seek to solidify its position as the most-prescribed Alzheimer's drug. While the ruling would cover only 20% of the Alzheimer's market and the drug is currently often prescribed along with its competitor, doctors may now be able to make a single prescription instead of two, and that can have a significant impact for both Eisai and the competition.

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Fool contributor Rich Duprey owns shares of Eisai but does not own any of the other stocks mentioned in this article. The Motley Fool has a disclosure policy.