What is the difference between 20 and 23? If you said three, you are off by millions—of dollars in sales, that is—at least from the perspective of Eisai, the manufacturer of donepezil (marketed as Aricept by Pfizer).

A little context helps make the maths clearer. Donepezil, the biggest player in the lucrative market for Alzheimer’s disease treatments, was a blockbuster, with over $2bn in annual sales in the United States alone. But the drug, first approved in 1996, had reached the end of the road: the patent expired in November 2010. Investors call this “going over the cliff,” an anxious reference to plummeting sales as market share is lost to generic competitors. Necessity, however, is the mother of invention. Just four months before the expiry of the patent, the US Food and Drug Administration (FDA) approved a new dose for moderate to severe Alzheimer’s disease: donepezil 23 mg. Is 23 an odd number? Not really, when you consider that you cannot get to 23 mg using the 5 mg and 10 mg doses that were going generic. The “new” 23 mg product would be patent protected for three more years.