Every time you pick up a prescription at the drugstore, you’re handed a set of instructions showing what the drug is for, how to use it, and its possible side effects.

But millions of people across the country take more than one medication at a time, and some of those drugs can interact in dangerous, even deadly ways.

Reporters at the Chicago Tribune set out to learn whether pharmacies in Chicago have been warning patients who’ve been prescribed risky combinations of drugs.

The newspaper’s findings, published last month, are disturbing: More than half of the pharmacies sold the combinations without warning customers, providing “striking evidence of an industrywide failure that places millions of consumers at risk.”

CVS, one of the nation’s largest pharmacy chains, failed 63 percent of the time. A competitor, Walgreens, did much better, but still didn’t warn customers almost a third of the time.

The investigation was a massive undertaking. First, reporters teamed with researchers to identify the right drug combinations to test. Then, a doctor working with the paper agreed to prescribe the drugs to the journalists, who brought the prescriptions to 255 independent and chain pharmacies in the Chicago area.

“Pharmacies are supposed to be this important cog in the healthcare industry,” Tribune reporter Sam Roe said. “They want to be looked upon as important. Yet, they’re behaving in many ways as glorified vending machines. They’re breaking the law to boot.”

Confronted with the findings, many of the pharmacies pledged to overhaul their procedures and put in safeguards. Just this week, Illinois Gov. Bruce Rauner introduced a proposal that would require pharmacists to tell patients about risky drug combinations. He also wants to launch a “mystery shopper” program to test compliance.

Listen to Roe and his colleague Ray Long tell how they did it on iTunes, Soundcloud or Stitcher.

Have an idea for The Breakthrough? Send us your suggestions — including which reporters we should talk to — at [email protected].