The cost of prescription medicines used by millions of people every day is about to plummet.

The next 14 months will bring generic versions of seven of the world's 20 best-selling drugs, including the top two: cholesterol fighter Lipitor and blood thinner Plavix.

The magnitude of this wave of expiring drugs patents is unprecedented. Between now and 2016, blockbusters with about $255 billion in global annual sales are set to go off patent, notes EvaluatePharma Ltd., a London research firm.

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