Dr. Jon Duke of Indiana University was trying to figure out why his patient’s blood platelets were abnormal. Could it be a side effect of one of the dozen drugs the man was taking, a number that is not uncommon among elderly people?

He began reading the label of each and every drug. “I was just overwhelmed,” Dr. Duke said. The lists of possible adverse reactions went on and on.

Now he knows why. In a new paper in the Archives of Internal Medicine, Dr. Duke and two colleagues report that the average drug label lists 70 possible side effects and some drugs list more than 500. “This was beyond even what I’d expected,” he said.

For anyone who has ever had to watch an entire Flomax commercial, the listing of a drug’s side effects is almost a joke. But the question is, why does the list continue to grow?