Payne tested positive for Phentermine after submitting a urine sample at a regional meeting in Swan Hill in Australia's southern state of Victoria on June 11.
The 31-year-old faced a stewards inquiry in Melbourne on Thursday after being stood down by racing stewards this week.
She pleaded guilty at the inquiry, which heard she had been prescribed the suppressant after a serious fall last year.
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Payne said she took "full responsibility" for the failed test and had not sought proper medical guidance on the usage of the suppressant.
“This is 100 percent my fault,” local media quoted her as saying after the hearing.
Payne had surgery after the fall during a ride in Mildura and had a long period of rehabilitation before returning to the track last September.
Payne shot to international fame in 2015 when she won Australia's richest and most famous horse race on 100-1 shot Prince of Penzance.
Her remarks after the race also went down in local sporting folklore, telling "chauvinists" in Australia's racing industry to "get stuffed".
Last week, she rode Kaspersky to fifth place in the Group One Queen Anne Stakes at Royal Ascot.
Payne's ban will expire on July 21.
(Reporting by Ian Ransom; Editing by Peter Rutherford)
