The cost of prescription medicines used by millions of people 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. By 2016, blockbusters with about $255 billion in global annual sales are set to go off patent, notes EvaluatePharma Ltd., a London research firm. Generic competition will decimate sales of the brand-name drugs and slash the cost to patients and companies that provide health benefits. Top drugs getting generic competition by September 2012 are taken by millions every day: Lipitor alone is taken by about 4.3 million Americans, and Plavix by 1.4 million. Generic versions of big-selling drugs for blood pressure, asthma, diabetes, depression, high triglycerides, HIV, and bipolar disorder are also coming by then.