Known in MMA circles as Cris Cyborg, the California State Athletic Commission fined Santos US$2,500, suspended her licence and nullified her Dec. 17 victory in San Diego against Hiroko Yamanaka.
Santos said on her website she unknowingly ingested banned substances from a dietary supplement she was taking to reduce weight for the bout.
"It was never my intention to obtain an unfair advantage over Hiroko," she wrote.
The CSAC did not specify in its statement the length of the suspension and its Sacramento office was closed Monday for Martin Luther King Day.
The suspension was applied retroactively to Dec. 16, 2011. It has been reported to be one year in duration.
Santos, a 27-year-old Brazilian, can appeal the suspension although she indicated on her website she would not. She's also been stripped of her Strikeforce women's featherweight title.
Santos won her Dec. 17 bout against Yamanaka by knockdown just 16 seconds into the first round, but the result has since been changed to a "no decision."
A drug test taken the previous day was analyzed by a World Anti-Doping Agency facility at the University of California and came back positive for stanozolol metabolites, according to CSAC.
"Our primary concern is for the health and safety of fighters," CSAC executive officer George Dodd said in the statement.
"Anabolic agents and other banned substances put not only the users of those agents at risk, but their opponents as well. The commission simply will not tolerate their use."