Along with Tobias, fellow Ukrainian Antonina Yefremova was charged after synthetic testosterone was found in their systems, while Bulgaria's Inna Eftimova tested positive for a synthetic growth hormone.
Jamaica's Shericka Williams qualified fastest in winning the second semifinal in 49.51 ahead of Russia's Antonina Krivoshapka (49.67), with Debbie Dunn of the U.S. claiming one of the two other spots up for grabs in 49.95.