(CNN) -- San Diego Padres fans remember Hall of Famer Tony Gwynn's hitting prowess: 3,141 hits, a .338 batting average and an intense focus at the plate.
What they probably don't remember is Gwynn using smokeless tobacco throughout his career -- a habit, Gwynn told ESPN in 2010, that he believed gave him cancer. Gwynn's use doesn't specifically come to mind because so many of his fellow Major League Baseball players had the same habit.
A survey administered in 1999 found that close to one-third of rookies starting in the major leagues were already regular smokeless tobacco users. More than two-thirds had tried smokeless tobacco. Other studies found similar rates -- about 30% -- in the majors in the '70s and '80s, though smokeless tobacco use among players began declining in the late 1990s (PDF).
But it wasn't until 2011 that MLB implemented rules related to smokeless tobacco products. Worried about the message it was sending to young fans, MLB collaborated with the Major League Baseball Players Association to prohibit teams from providing tobacco to players. Players cannot have tobacco tins in their uniforms or do interviews while using chewing tobacco.
Yet the players' union stopped short of banning tobacco use on the field.
Smokeless tobacco is the general term for two products: chewing tobacco and snuff. Though snuff can be inhaled, most users place smokeless tobacco in their mouth and suck on it, periodically spitting out the juices.
Smokeless tobacco contains 28 carcinogens, according to the Centers for Disease Control and Prevention, and is a known cause of oral cancer.
Smokeless tobacco is primarily a male habit, the CDC says. In 2012, approximately 11% of high school boys used smokeless tobacco; only 1.5% of girls did. Nationally, an estimated 6% of adult men use smokeless tobacco, compared with just 1% of adult women.
"Current use of smokeless tobacco is about half of what it was in the mid-1990s," the CDC Youth and Tobacco use website states. "However, only a modest decline has occurred since 2010 and no change occurred between 2012 and 2013."
Gwynn was diagnosed with cancer in 2010. ESPN reported at the time that the then-50-year-old faced chemotherapy and radiation to fight the "slow-moving but aggressive" cancer in his salivary gland.
There is no scientifically established link between smokeless tobacco and salivary gland cancer, according to the American Council on Science and Health. Doctors don't know what causes salivary gland cancer, but the Mayo Clinic lists old age and radiation as known risk factors.
"The kind of chewing tobacco Gwynn used does have some risk of oral cancer, but not parotid (salivary) gland cancer, which is the kind that took his life at such a young age," said Dr. Gilbert Ross, the council's medical director.
Gwynn isn't the only former ballplayer to battle oral cancer. Most notably, Babe Ruth, Brett Butler and Bill Tuttle were all diagnosed after years of chewing tobacco use. Butler became a passionate advocate against tobacco after he returned to the field following treatment, according to the Oral Cancer Foundation.
Still, players say the habit is hard to break.
"I use it as a stimulator when I go to hit," Boston Red Sox star David Ortiz told the Boston Globe. "But the minute I finish my at-bat, I spit it out. It keeps me smooth and puts me in a good mood. I don't do it in the off season. I don't really like it that much, to be honest with you."
Nicotine, the addictive substance found in cigarettes, occurs naturally in all tobacco.
"Over time, a person becomes physically dependent on and emotionally addicted to nicotine," according to the American Cancer Society.
Those who do try to quit experience withdrawal, often for weeks after their last spit or chew. Withdrawal symptoms can include irritability, dizziness, depression, headaches and weight gain, according to the American Cancer Society.