Russia's Yuliya Efimova has been temporarily suspended after testing positive for meldonium.
The 23-year-old four-time breaststroke world champion returned a positive result during an out-of-competition test.
The London 2012 bronze medallist could now be facing a life ban having previously failed a drugs test in 2014.
Meldonium was added to the World Anti-Doping Agency's banned list on 1 January 2016.
So far more than 100 athletes in various sports have tested positive for the drug, including Russia's grand slam-winning tennis player Maria Sharapova.
A statement on Efimova's suspension said: "The All-Russian Swimming Federation (ARSF) informs that we received documents from the International Swimming Federation (Fina) which confirm that the sportswoman has been temporarily suspended from competition due to the possibility of breaking anti-doping regulations.
"In order to clarify information concerning the current situation as quickly as possible, ARSF president Vladimir Salnikov will meet with the heads of the Russian swimming team and the Russian Sports Ministry."
US-based Efimova was banned for 16 months by Fina after traces of the anabolic steroid DHEA (dehydroepiandrosterone) were found in her system at an out of competition test in Los Angeles in 2013.