By now, you’ve seen the headlines: “EPO Doesn’t Boost Performance.” That’s right: In the British Journal of Clinical Pharmacology, researchers reported that “there is no scientific basis to conclude rHuEPO has performance enhancing properties in elite cyclists.” But should you believe them?

We could answer that question with anecdotal evidence—Outside’s own Stuart Stevens dabbled with EPO for a story and found it to be incredibly effective—but we turned to Dr. Michael J. Joyner, a physician-researcher and one of the world’s leading experts on human performance and exercise physiology for his take.

“This thing is nuts,” he says. “It’s correct about only one thing: There are essentially no studies on really, really elite people.” In other words: Don’t go reinstating Lance Armstrong into the pantheon just yet. 

So where did the study go wrong? It goes back to basic exercise physiology and the science of how races are won, Joyner says. The researchers don’t understand the relationship between VO2 max—your body’s maximum ability to consume oxygen, or your “performance ceiling”—and other metrics like lactate threshold, a key predictor of endurance performance, Joyner says.

(In the context of cycling, a five-minute-long pursuit would best correspond to a VO2 max effort, while a 60-minute time trial would closely match your lactate threshold.)

After reviewing the existing literature on EPO, the researchers found that a doping regimen can lead to between a 7 and 9.7 percent increase in VO2 max, with an “increase in performance estimated by a time-to-exhaustion test of ... 9.4 percent (versus 1.5 percent in placebo-treated subjects) and 16.6 percent in trained subjects.”

Here’s how: EPO is a naturally-produced hormone that regulates red blood cell production. Red blood cells happen to carry oxygen, the delivery of which is a limiting factor in endurance performance. More EPO means more red blood cells which means a faster you.

The researchers actually pointed all of this out, but they weren’t convinced that EPO would have an effect on race-day performance because “cyclists only work a small amount of time at their peak intensities,” they wrote.