The violation was Hargrove's second since August, when he was suspended without pay for the first four games of the regular season.
A third violation triggers an automatic one-year suspension, and Hargrove must apply to commissioner Roger Goodell for reinstatement.
The NFL did not disclose how Hargrove violated the policy. Violations can result from positive tests, failure to cooperate in the testing program or failing to comply with a treatment plan.
Hargrove's agent, Philip Williams, was out of the country and not immediately available for comment.
The Bills declined comment beyond issuing an NFL release announcing the suspension, which came three weeks after Buffalo (7-9) completed its season.
Though Hargrove has said he hoped to re-sign with the Bills, his future in Buffalo was already uncertain. He has completed his one-year contract and was eligible to become a free agent this offseason.
In 12 games last season, Hargrove finished with 46 tackles and 1.5 sacks, and proved to be a capable backup behind starters Aaron Schobel and Chris Kelsay.
Outgoing and well-liked by his Bills teammates, Hargrove has been unable to stay out of trouble during a five-year NFL career, including the past two with Buffalo.
A week before he was suspended in August, Hargrove was involved in an altercation with police outside a Rochester nightclub during an off-day at Bills training camp in nearby Pittsford. That resulted in a $300 fine, and Hargrove was ordered to perform 200 hours of community service after pleading guilty to disorderly conduct.
Hargrove has said he regretted his actions.
Copyright 2007 by The Associated Press