OWINGS MILLS, Md. -- Cornerback Jimmy Smith was suspended four games for violating the NFL policy on performance-enhancing substances, which continues a bad trend for the Baltimore Ravens.

Since the start of the 2012 season, the Ravens have had eight instances of a player being suspended for a PED violation. That ties Baltimore with the New York Giants for the most in the NFL over that span, according to ESPN Stats & Information.

John Harbaugh believes coaches can only monitor players to a certain degree. The issues with PEDs are mostly addressed by the league as well as trainers and agents, who are very involved in what supplements players can and can't use.

"To me, it's the player's responsibility to do it," Harbaugh said. "If they want to play, they're going to be very vigilant about that and do a good job with it. That's the way most coaches are, and that's really all you can do. It's pro football."

In total, Ravens players have been suspended 41 games for PEDs over the last six years. The players disciplined range from someone who never played a down for Baltimore (defensive lineman Ryan McBean) to one of the top defenders in team history (defensive tackle Haloti Ngata). It included repeat offenders such as cornerback Asa Jackson and tight end Nick Boyle.

This year, Smith and running back Kenneth Dixon both will serve suspensions while being on injured reserve. Dixon received a four-game suspension for PEDs before suffering a season-ending knee injury before the start of training camp. Smith, who tore his Achilles on Sunday, will lose four weeks of pay while serving his suspension on IR.

"The guys are very vigilant, for the most part, on that, and sometimes they make mistakes," Harbaugh said. "There's a broad array of different things that guys can get involved with, different supplements and things like that. I'm really not well-versed on it. It's not something that I spend a lot of time studying. I was not very good at chemistry in high school. I didn't like it, and I was happy to sneak by with my ‘C.'"