"The substance was a medication that I should have gotten clarification on before taking," Spikes said in a statement. "It was not a performance enhancer or an illegal drug. The integrity of the game is very important to me. I understand the league's ruling and apologize to my teammates, the fans and the Patriots organization for this mistake."
Medication designed to treat attention deficit disorder, which included amphetamines, caused Spikes to test positive for performance enhancers, according to a report from Comcast Sports New England.
Spikes' suspension begins immediately, with Spikes eligible to return after the Patriots' final regular-season game Jan. 2.
When reached by ESPNBoston.com for comment, Spikes' agent, Terry Watson, said, "I support Brandon's statement from yesterday."
Asked specifically about reports that the medication that caused the positive test was designed to treat attention deficit disorder, Watson said, "No comment."
Spikes does not plan to appeal the suspension. He will serve it immediately to return in time for the playoffs.
The rookie from Florida is a first- and second-down player in the team's 3-4 alignment, a strong downhill player against the run. The second-leading tackler on the team, his presence will be missed against teams that feature a physical running game.
In his absence, the Patriots are likely to turn to third-year player Gary Guyton, who was a starter at strongside inside linebacker in 2009. Guyton has been the team's top sub linebacker this season, replacing Spikes in passing situations.
Rookie free agent Dane Fletcher also would move up one spot on the depth chart.
With the Patriots playing more sub defenses in recent weeks, Spikes' playing time has dipped a bit. But his production per play was evidenced against the Jets on Monday night when he finished with seven tackles in 23 snaps.
Spikes does not play on special teams units, so there is limited impact in that area.
The Patriots also announced they signed defensive back Chevis Jackson on Friday. A third-round pick of the Falcons in 2008, the Louisiana State product was released by Atlanta in September, caught on with Jacksonville and played two games before being cut Nov. 17.
Information from ESPNBoston.com Patriots writer Mike Reiss was used in this report.