The arrest of four football players on the Texas Christian University (TCU) team this week on suspicion of selling marijuana points to an increasing problem in college athletics, says the Vice President of the National Center for Drug Free Sports. The four TCU students were among the 17 students arrested this week as part of a six-month drug sting.

Andrea Wickerham told the Associated Press the arrests are not an isolated incident. They came one month after the National Collegiate Athletic Association (NCAA) said 22.6 percent of 20,474 student-athletes who participated in an anonymous survey in 2009 admitted to using marijuana in the previous year—an increase from 21.2 percent in 2005.

The 2009 survey found 26.7 percent of football players and 22 percent of men’s basketball players said they used marijuana in the previous year. The survey, which has been conducted every four years since 1985, has always found alcohol to be the number-one substance of choice, with marijuana coming in second. The NCAA tests for marijuana at championship events and football bowl games, but not in its year-round testing program, according to the AP.

In 2009-2010, 1,645 student athletes were tested, and 4.3 percent were found positive for marijuana, up from 1.6 percent the previous year.

“You want to test often enough so athletes truly believe they have a likelihood of being selected,” says Wickerham, whose group administers drug tests for more than 250 colleges as well as the NCAA. “If you’re only doing it once a semester, or if you do it only when you hear about a bad event, that’s not a huge deterrent over time.”