They say, it's like being chased by a car, it's more exhilarating than any professional fútbol game, and for some the thrill is just too enticing to pass up.
For your average daredevil, risking your life once at the San Fermin Festival is enough. Then there are the veterans, who return year after year to run with the bulls, unable to kick their fix.
The tradition immortalized by Ernest Hemingway in "The Sun Also Rises" is the ultimate rush for this small club of regulars, who prepare obsessively for the electrifying and sometimes shocking dash through Pamplona's old quarter, held every year in the second week of July.
"First you're hooked by the tradition, then it gets you as a fan and, finally, you get to the point where it's an extreme addiction, where you need to be running with the bulls," said Juan Pedro Lecuana, a 38-year-old father of four who has been coming back every year since 1989.
At 8 a.m. sharp each morning from July 7-14, six half-ton fighting bulls are freed from a corral to rumble after thousands of runners down a fenced route of about 800 meters (875 yards), ending up in a bullring where they will face matadors and certain death by afternoon.
Most runners dash through the cobblestoned streets for about 50 meters (yards) before jumping out to safety.
For the humans, any slip-up could signal death, so preparing properly for an event that combines elements of hurdles and rugby scrum can never be taken lightly -- a lesson taken to heart by the veterans but often ignored by thrill-seeking tourists.
Fifteen people have been killed since record-keeping started in 1924 with Daniel Jimeno Romero the last in 2009.
Chicago-native Rick Musica has missed only four runs over 13 years. He watches hundreds of videos throughout the year to gain better insight into surviving the obstacle course.
"On one hand, you have this sheer exhilaration and on the other, sheer utter terror ... balancing those emotions is the key (and) it's not always easy. Actually, it's never easy," the 45-year-old Musica wrote in an email.
"To see these magical beasts thumping through the narrow streets is something that defies logic and something that is unlike anything I have ever done in my life. ... It is truly a celebration of life."
The dos and don'ts for the two- to three-minute dash are simple: don't yell at the animals, carry a rolled up newspaper for a handy distraction and, most importantly, if you fall don't get up. American runner Matthew Tassio, 22, did just that in 1995 and was immediately killed after being charged.
Being physically fit is important, and veteran runners fine-tune through various physical activities, from running to swimming in the month leading up to San Fermin.
Lecuano, who works in the auto industry, has suffered broken ribs, cuts, stitches. He'll remember his last July 11 not for Spain's World Cup victory but for his being gored in the leg. Being bedridden for two months meant added weight, and a harder return.
"Let's see what goes through my head on the seventh," Lecuano cautioned.
Pamplona city council has even released a handy application for the iPhone called Bull Runner Trainer, which offers advice, tips and a full database of results for each run dating back to 1980. It also allows people to select a stretch of the course, choose a distance to the bulls, and after a 50-yard sprint, measures how well people would fare.
Peter Milligan gets in the groove by dashing through the busy city streets around his Cherry Hill, New Jersey, home and avoiding pedestrians while glancing over his shoulder to mimic what he'll need to do in Pamplona. Looking back is nearly as important as forward, as well as practicing full-out sprints.
"It's like being chased by a Honda Civic," Milligan said. "I try to run looking over my shoulder, to know what's coming -- keep my knees and feet in good shape."
Milligan, a lawyer, has even visited ranches to observe the bulls, which he says gave him a better sense of their instincts.
"Once you do it you either never want to do it again or you want to come back every year for the rest of your life," Milligan said over the telephone in an excited voice.
"We fell in with experienced people, people who really helped us out, gave us sound advice. We knew we couldn't just sit on sidelines as it's one of the few things in life you don't have to sign up for, there's no rules like everything here in North America."
Cesar Cruchaga, former captain of the Osasuna soccer club in Spain's first division, was only able to recently return to running after ending his professional career. Like Lecuana, his first run was at age 15.
"It's a very different feeling to scoring any goal ... so intense it beats any game of football I played," the Pamplona native said.
"That sensation that death is so near -- there is nothing comparable to that."
Based on reporting by the Associated Press.