|National team||United States|
|Born||May 30, 1995|
|Height||1.778 m (5 ft 10.0 in)|
|College team||University of Texas|
She won bronze in the 400-meter individual medley at the 2016 Short Course World Championships. She had originally finished 4th, but was elevated to bronze, along with teammate Ella Eastin being elevated to silver, when Ánh Viên Nguyễn was disqualified from second place.
Cox qualified to swim the 200-meter individual medley at the World Championships after finishing second at the 2017 US Nationals. She went on to win the bronze medal in Budapest with a time of 2:09.71. She also won a gold medal as a prelim swimmer of the 4x200 freestyle relay.
Cox was banned from competition for six months after a urine sample taken in February 2018 tested positive for trimetazidine. FINA initially reduced her suspension from four years to two years because of Cox's testimony that she did not knowingly ingest the performance-enhancing drug, but would not reduce it further without evidence of the source of the trimetazidine. Upon analysis of both opened and sealed bottles of Cooper Complete Elite Athletic multivitamins, the Court of Arbitration for Sport determined that the multivitamins were the source, and reduced Cox's suspension to six months. The suspension expired on September 3, 2018. However, as United States Swimming World Championship trials occurred during her suspension, Cox was not able to qualify for the 2019 World Aquatics Championships.
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- Gibbs, Robert (2018-08-31). "Madisyn Cox's Suspension Reduced to Six Months after Trimetazidine Detected in Multivitamin". SwimSwam. Retrieved 2018-09-03.
- "Madisyn Cox Eligible to Compete After Source For Banned Substance Found in Multivitamin". Swimming World News. 2018-08-31. Retrieved 2018-09-03.