It's called the "number needed to treat," or NNT. As Aaron Carroll explains below, it's how many people need to take a particular treatment for one of them to receive the advertised benefit. For example, if we imagine a treatment for the common cold with an NNT of 5, that means for every person whose cold was successfully treated, 4 people took the medicine for no benefit.
Sounds simple, right? What's amazing is just how high this number can be. Even very famous and well-accepted treatments often have NNTs in the hundreds or thousands. Watch the video for the full explanation. --Ryan Cooper