Brazil international midfielder Fred has vowed to prove his innocence after he tested positive to a banned substance.
The 22-year-old issued a statement on Friday denying he had used the diuretic hydrochlorothiazide, Xinhua news agency reported on Saturday.
"I trust in my innocence and I am going to prove it to the proper authorities," Fred said.
Earlier, Brazil's Football Federation (CBF) confirmed the Shakhtar Donetsk player had tested positive to the substance during the Copa America in Chile in June.
Normally used to control hypertension and cardiovascular problems, hydrochlorothiazide is also a known masking agent for performance-enhancing drugs.
Meanwhile, Fenerbahce has asked UEFA to ban Fred immediately from European matches, pending the outcome of a B sample.
Fred played in Shakhtar's Champions League third qualifying round first leg tie against Fenerbahce on Tuesday, which ended 0-0. Shakhtar will host the return leg on August 5.
UEFA's control, ethics and disciplinary body are expected to make a decision on Fred's availability on August 3.
