The Italian, a Roland Garros finalist in 2012, was suspended for two months in August 2017 by an independent tribunal of the Italian Tennis Federation after she failed an out-of-competition test, administered at her family home, last February.
Errani said she had consumed letrozole, which can have a "marginal" anabolic effect and mask the use of testosterone, unknowingly when her mother's cancer medication somehow found its way into a family meal.
After the original ruling, both Errani, who wanted the ban overturned, and the Italian anti-doping agency, who wanted it increased, launched appeals.
On Monday, CAS issued a statement saying it accepted that the letrozole "was medication taken by her mother that found its way into the family meal prepared by the athlete's mother and eaten by the entire family".
CAS said that Errani had to be held responsible for her mother's mistake and while this amounted to a "light degree of fault", increased her ban to 10 months. The two months that Errani served last year are deducted from that total.
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