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. 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 also are coming by then.

The flood of generics will continue for the next decade or so, as about 120 brand-name prescription drugs lose market exclusivity, according to prescription benefit manager Medco Health Solutions Inc.

“My estimation is at least 15 percent of the population is currently using one of the drugs whose patents will expire in 2011 or 2012,” says Joel Owerbach, chief pharmacy officer for Excellus Blue Cross Blue Shield, which serves most of upstate New York.

Those patients, along with businesses and taxpayers who help pay for prescription drugs through corporate and government prescription plans, collectively will save a small fortune. That’s because generic drugs typically cost 20 percent to 80 percent less than the brand names.

Doctors hope the lower prices will significantly reduce the number of people jeopardizing their health because they can’t afford medicines they need.

Dr. Nieca Goldberg, director of The Women’s Heart Program at NYU Langone Medical Center in Manhattan, worries about patients who are skipping checkups and halving pills to pare costs.

“You can pretty much tell by the numbers when I check the patient’s blood pressure or cholesterol levels,” that they’ve not taken their medications as often as prescribed, she says.

Even people with private insurance or Medicare aren’t filling all their prescriptions, studies show, particularly for cancer drugs with copays of hundreds of dollars or more.

The new generics will slice copayments of those with insurance. For the uninsured, who have been paying full price, the savings will be much bigger.

Generic medicines are chemically equivalent to the original brand-name drugs and work just as well for nearly all patients.

When a drug loses patent protection, often only one generic version is on sale for the first six months, so the price falls a little bit initially. Then, several other generic makers typically jump in, driving prices down dramatically.