NEW YORK – St. Louis Rams linebacker David Vobora has been suspended for four games after testing positive for a banned substance.

The NFL said Vobora had violated the league's policy on performance-enhancing substances, but provided no other details in a statement on Tuesday.

Vobora's agent Marc Lillibridge said on NFL.com that his client had failed a doping test after taking a supplement tainted with the banned substance.

Lillibridge said Vobora was planning to sue the company that made the supplement and that a toxicology report showed the product had been tainted.

It was unclear what the substance was and when Vobora failed the doping test. The NFL was not available to provide comment.

Rams head coach Steve Spagnuolo said Vobora's situation was "unfortunate."

"He has been a vital part of our team and we are disappointed that we will be without him for the next four games," he told the Rams' website (www.stlouisrams.com).

"However, we respect the league's decision in this matter."

Vobora, who made his second career start on Sunday in the Rams' home loss to Green Bay and ranks third on the team with eight tackles, can return to the active roster on October 26 and play against the Detroit Lions on November 1.