That's according to several studies.
"If someone drinks five to ten cups of decaffeinated coffee, the dose of caffeine could easily reach the level present in a cup or two of caffeinated coffee," says researcher Bruce A. Goldberger, PhD, of the University of Florida, in a news release.
In a study administered by Journal of Analytical Toxicology, researchers found that only one "decaf" coffee out of the ten tested was actually caffeine-free: decaffeinated Folgers Instant.
In another study by ConsumerReports.org, secret shoppers bought 36 cups of decaf from six locations of Burger King, Dunkin' Donuts, McDonald's, Seattle's Best Coffee, 7-Eleven, and Starbucks and tested them for caffeine content. To quote from the study:
More than half of our decafs had less than 5 mg of caffeine, but some had quite a bit more. One of the six cups from Dunkin' Donuts had 32 mg; one from Seattle's Best had 29 mg; and one from Starbucks had 21 mg. Levels of caffeine in the decaffeinated coffees we tested varied within chains, but in our sample, McDonald's decaf consistently had less than 5 mg.Reference point: an 8-ounce cup of regular drip-brewed coffee contains about 85 milligrams of caffeine.
Overall, caffeine amounts ranged from around 1/10 to a little over 1/3 of that in regular coffee.
It's a lesson learned: decaffeinated ≠ caffeine-free.