The UFC Anti-Doping Policy requires fighters who are coming out of retirement, such as former heavyweight champion Brock Lesnar, to notify the promotion in writing four months in advance of an intended return to competition, and to make themselves available for random testing for four months.
Lesnar on Friday agreed to terms and signed his bout agreement to fight Mark Hunt at UFC 200 on July 9 in Las Vegas. He entered into the testing pool on Monday. He thus would not be able to fight, though the clause also covers situations such as Lesnar.
The reason is for fairness and when it is impossible to comply.
Paragraph 5.7.1 of the testing policy covers both the four-month requirement for athletes who come out of retirement as well as the ability to grant an exemption.
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5.7.1 An Athlete who gives notice of retirement to UFC, or has otherwise ceased to have a contractual relationship with UFC, may not resume competing in UFC Bouts until he/she has given UFC written notice of his/her intent