Guerrero, who plays as a striker for Brazilian side Flamengo, was initially banned for one year after traces of the banned drug were found in a sample following a World Cup qualifier against Argentina on October 5 last year.
The CAS made a decision to increase Mr Guerrero's suspension from 6 to 14 months, starting today, with the period of provisional suspension of 6 months already served by Mr Guerrero being credited against the total period of ineligibility to be served. It reduced the ban in a fast-tracked appeal last December which clearing him to play at the World Cup.
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But CAS have increased Guerrero's suspension from six to 14 months on the basis that he did bear "some fault or negligence" for the positive test, and must therefore face a ban of between one and two years under FIFA's own anti-doping rules.
Guerrero, who has denied wrongdoing throughout, had also appealed to CAS to request the ban be annulled altogether.
Despite acknowledging Guerrero had no intention of taking a banned substance - his lawyer argued the stimulant was consumed by drinking contaminated tea - CAS ruled in line with WADA's wishes.
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The court has since announced that their judging panel "also accepted that he did not attempt to enhance his performance by ingesting the prohibited substance".
CAS said that Guerrero bore "some fault or negligence, even if it was not significant, and that he could have taken some measures to prevent him from committing an anti-doping violation".
He was provisionally banned by Federation Internationale de Football Association in November, ruling out of the qualifying playoff against New Zealand which Peru won 2-0 on aggregate.
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Guerrero scored his first goal on Sunday since the suspension, a header in Flamengo's 3-2 defeat at Chapecoense in the Brazilian championship. The three points helped Peru qualify for the playoffs- where they beat New Zealand to make it to Russia- at the expense of Chile.