Former Chelsea and Portugal playmaker Deco is facing a 30-day automatic suspension after his "B'' sample confirmed a positive test for a diuretic.
Deco tested positive for furosemide, a diuretic which can help hide performance-enhancing substances. The test came after Fluminense's 2-0 win over Boavista on March 30.
If found guilty by Brazil's sports tribunal, the 35-year Deco faces a suspension of up to two years.
Fluminense said Friday that Deco has already hired lawyers to defend him. The player vehemently denied any wrongdoing and said the result likely stemmed from contaminated vitamins.
He said before the "B'' sample results were released that he would "go all the way" to prove his innocence.