Visitors to haunted houses or riders of roller coasters will often tell others that they were “scared to death” during their experiences. But obviously they weren’t, if they’re telling you about it. Which raises the question: Is it even possible to be scared to death?

It is definitely possible to be so scared that you die, but it’s very rare. According to ASAP Science, when a person is frightened by something, his body’s fight or flight response kicks in. Adrenaline is released, which makes the heart beat more quickly, sending more blood to the muscles. The result? That very scared person temporarily becomes a stronger and faster version of himself. (Fight or flight also triggers pupil dilation and slows digestion, among other effects.)

“All of this increases the chances of succeeding in a fight or running away from, say, an aggressive jaguar,” neurologist Martin A. Samuels told Scientific American in 2009. “This process certainly would be of help to primitive humans, but the problem, of course, is that in the modern world there is very limited advantage of the fight-or-flight response. There is a downside to revving up your nervous system like this.”

In a typical fight or flight response, the effects are only temporary, and the heart can handle it just fine. But in large doses, adrenaline isn’t good for you. In fact, it’s toxic. According to Samuels, adrenaline hits the receptors in the heart-muscle cells, which makes the calcium channels in the membranes of the cells open up. “Calcium ions rush into the heart cells and this causes the heart muscle to contract,” Samuels said. “If it's a massive overwhelming storm of adrenaline, calcium keeps pouring into the cells and the muscle just can't relax.”

When the system that regulates the rhythm of the heart is overloaded with adrenaline, the organ can go into rhythms that, Samuels said, “are not compatible with life. If one of those is triggered, you will drop dead.” He pinpointed ventricular fibrillation—which the American Heart Association calls “the most serious cardiac rhythm disturbance” in which “the lower chambers quiver and the heart can't pump any blood, causing cardiac arrest”—as the one most likely to cause death from fear. It’s been established that fear lowers the threshold for arrhythmia in a normal heart, and if there’s some blockage, fear can triple your rate of ventricular fibrillation.

It’s not just fear that can cause a person to drop dead, either: Samuels told NPR in 2012 that “a powerful positive emotion can do it, as well. I have an example of a guy who hit a hole in one, he played golf his whole life and hit a ball over a rise and didn't see where it went. He and his partner went over and looked down on the green, and the ball was in the hole. And he said ‘wow, I hit a hole in one, I can die now,’ and he did.”

People are more at risk of dropping dead from fright (or happiness) if there’s already damage to their hearts. “A person who is walking around with a 50 percent narrowing of the arteries may never have symptoms, but if they're held up at gunpoint or narrowly miss an auto accident, their adrenaline levels can rise and destabilize that plaque,” Ilan Wittstein, a cardiologist at Johns Hopkins Hospital in Baltimore, told the Wall Street Journal in 2012. “That ruptured plaque can cause a blood clot to form and now they have a 100% blockage.” But healthy people can be scared to death, too.