Cris "Cyborg" Justino, the top-ranked female featherweight, has been informed by the U.S. Anti-Doping Agency of a possible doping violation, the UFC said in a statement.
The promotion said it was formally notified by the USADA of the potential violation Thursday. It stems from an out-of-competition collection taken Dec. 5.
Justino last fought Sept. 24, beating Lina Lansberg by TKO in Round 2 as part of a UFC Fight Night in her native Brazil. Justino said she had to cut 25-plus pounds for the fight, which was contested at 140 pounds since the UFC didn't have a 145-pound division at the time.
In a statement, Justino said that she was prescribed the medication, Spironolactone, to address hypertension and to prevent ovarian cysts after she "had an electrolyte hydroelectric imbalance" and "significant loss of potassium" following that September fight. She said that she has a history of cysts and when she arrived at the doctor she was bloated, very weak and had a low white blood cell count.
Justino added that her doctor is very familiar with USADA testing and told her there were no additional steps she had to take for approval of her treatment. She is in the process of applying for a retroactive therapeutic use exemption and is fully cooperating with USADA.
Justino apologized to her fans.
"For my fans who are disappointed in the news, I am sorry," she wrote.
She added: "It is my hope that my experiences will continue to bring awareness to the dangers of extreme weight cutting."
In December 2011, the 31-year-old Justino tested positive for an anabolic steroid after a knockout win as the Strikeforce featherweight champion. She blamed the positive test on a diet supplement she received from a former coach.
Justino (17-1) made her UFC debut in May, beating Leslie Smith in 81 seconds at UFC 198. Four months later, she beat Lansberg.
Earlier this month, the UFC passed over Justino for the first 145-pound female fight in promotion history. She said she felt "saddened" and "disrespected" by the UFC's decision to book Holly Holm and Germaine de Randamie instead.
UFC president Dana White disputed Justino's conclusions by saying the promotion made Justino three offers for the inaugural title fight. After she declined all of them, White said the promotion had to move forward.
Reached by TMZ on Thursday, White addressed Justino's potential doping violation and her recent negotiations with his promotion.
"First fight she had eight weeks to get ready. Second fight she had 11 weeks to get ready and the third fight she just turned down," White told TMZ.
"I thought it was just strange that she was turning down all these fights ... not so strange now."
Justino, in the statement, said she was given a medical suspension by her doctor "not to enter into competition style training or weight cutting practices" during the period of recovery from taking Spironolactone, which is why she declined the 145-pound fight Feb. 11.
Information from ESPN's Will Cain and Brett Okamoto was used in this report.