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A two-year investigation led by the Chicago Tribune revealed a hazardous trend involving pharmacists dispensing dangerous drug pairings without warning patients. The newspaper reported that 52 percent of the 255 pharmacies involved in the investigation sold the medications without mentioning the potentially deadly consequences of interactions to consumers.

For their tests, the newspaper sent 15 reporters into various CVS, Walgreens, Wal-Mart, Kmart and Costco pharmacies, as well as local independent pharmacies, to fill one prescription for clarithromycin, common antibiotic, and another for simvastatin, a popular anti-cholesterol drug.

The newspaper reported that when taken alone the drugs are safe, but once combined, patients are at risk of a breakdown in muscle tissue that could lead to kidney failure or potential death. The investigation was conducted in the Chicago area, Indiana, Wisconsin and Michigan. According to the Tribune, Illinois regulation requires pharmacists who identify a dangerous drug pairing to contact the prescribing doctor to verify the order and then alert the patient.

The Tribune’s report found that the CVS pharmacists involved in the investigation failed to warn patients of the potential interaction 63 percent of the time. While Walgreens pharmacists failed to warn patients 30 percent of the time, Kmart pharmacists failed 60 percent of the time and Wal-Mart in 43 percent of tests. Costco pharmacists also failed in 60 percent of the tests, the Tribune reported.

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The pharmaceutical chains had an overall 49 percent failure rate, with Walgreens, CVS, Kmart and Wal-Mart vowing to address the issue with staff and technicians as a result of the newspaper’s published investigation.

“There is a very high sense of urgency to pursue this issue and get to the root cause,” Tom Davis, CVS’ vice president of pharmacy professional services, told the Tribune.

Though the big-chain pharmacies scored poorly, the Tribune included independent pharmacies as well, which missed risky drug interactions 72 percent of the time.

“Anytime there’s a serious interaction, there’s no excuse for the pharmacist not warning the patient about that interaction,” Carmen Catizone, executive director of the National Association of Boards of Pharmacy, told the Tribune.

The report revealed that while the pharmacists may have failed to alert patients, a number of safety precautions put in place to prevent such oversight is also being ignored. The Tribune found computer alert systems either ignored the issue or were broken, and an emphasis was placed on faster production over safety.

One Wal-Mart pharmacist told the Tribune she fills 200 prescriptions in a nine-hour shift.

“Every prescription is timed,” Deepak Chande, former head pharmacist at a CVS, told the Tribune. “And this is the worst of the pharmacist’s nightmares.”

According for the Centers for Disease Control and Prevention (CDC), the sale of prescription opioids have quadrupled since 1999, and so have death rates. More than 165,000 people have died from overdoses related to prescription opioids from 1999 to 2014.