Story highlights A new study projects that $3 billion worth of the most common cancer medicines will go to waste this year in the U.S.

Drugs are wasted because manufacturers sell them in vials that are not practical to reuse

Experts urge policies requiring pharmaceutical companies to package drugs differently or buy back unused drugs

(CNN) Most shoppers wouldn't buy a gallon of milk if they thought they could only drink a pint's worth. So why are hospitals and clinics buying larger amounts of cancer therapies than they can use, forcing them to throw these expensive drugs in the trash?

The answer, according to a new study, is that many of the top cancer drugs in the United States are only available in single-dose vials, and often that dose is more than the average patient needs. The result is an estimated $3 billion in overspending in the health care system each year, a cost that gets passed on to taxpayers and patients.

In one of the more egregious examples, the pharmaceutical company Merck sells a melanoma drug, pembrolizumab, called Keytruda, in vials that contain 100 milligrams of the drug. A 150-pound patient gets about 140 milligrams of Keytruda, meaning that more than half of the second vial would not be used. Although some of the leftover drug might go to another patient, the study determined that on average, 21% of the drug, costing about $200 million, goes to waste.

The problem with Keytruda tipped researchers off to look at the amount of waste among the 20 most common drugs used to treat a range of cancers, including breast, prostate, skin and colorectal. Waste was rampant.

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