Rostov assistant coach Vitaly Kafanov has denied rumours that his players are taking meldonium.
FIFA drug testers arrived unannounced at the club on Thursday to collect samples amid suspicions of banned substance use during Rostov's surprise pursuit of the league title.
Rostov only remained in the Russian top flight after a relegation playoff last year, but have surprisingly been involved in the title race this season and were tested after beating Dynamo Moscow 3-1 to move within two points of league leaders CSKA Moscow with two games of the season remaining.
"I don't believe that the guys were taking banned substances," Kafanov told Russian newspaper Sport Express. "We don't have money for even simple medicine."
Rostov's financial difficulties have made their success even more remarkable, with the future of the club still in doubt despite this season's on-field success.
FIFA medical chief Jiri Dvorak said Rostov's players were "absolutely" compliant with his drug-testing team.
Meldonium was banned on Jan. 1 by the World Anti-Doping Agency. There have been more than 170 failed tests in various sports and countries -- many in Russia -- since the drug was prohibited.
The drug, which is typically prescribed for heart conditions, had been widely used as a supplement by athletes in Eastern European countries to increase blood flow and improve exercise capacity.
Seven of Rostov's starting 11 players on Thursday were born in Russia.