Merriman, 22, was the NFL Defensive Rookie of the Year last season and has contributed 5.5 sacks and an interception this season for the Chargers. On Sunday in Kansas City he collected eight tackles, including seven solo.
In recent weeks, Merriman had been garnering attention for being the best linebacker in the NFL.
The news comes at a time when fellow linebacker Shaun Phillips is sidelined with a calf injury. Phillips, who injured himself on Sunday in Kansas City, could miss two to four weeks.
First-time offenders of the NFL's steroid policy receive a minimum four-game ban, the equivalent of one-fourth of the regular season. A second positive test fetches eight games, and a third positive gets a yearlong suspension.