Guerrero was tested following Peru's 0-0 draw in its World Cup qualification match against Argentina in Buenos Aires on 6 October 2017. On 3 November 2017 he was notified that he had tested positive for the cocaine metabolite benzoylecgonine and provisionally suspended. On 31 May 2018 he was deemed eligible to play for Peru in this summer's World Cup in Russia by the Swiss Arbitral Tribunal who placed a stay on his 14 month ban pending his appeal. The decision was not contested by WADA.

A FIFA Disciplinary Committee heard the case at first instance and accepted Guerrero's explanation that he had consumed tea that included coca leaf extract. On 7 December 2017 the Committee issued its decision and imposed a one year ban.

Guerrero appealed the decision to the FIFA Appeal Committee. His defence was based on the fact that coca leaves are a relatively common ingredient in teas in Peru. His lawyers introduced some very interesting evidence in this regard – the results of CT scans on the 'Children of Llullaillaco', three five hundred year-old frozen Inca mummies found, in 1999, near the top of the twenty-two-thousand-foot-tall volcano in Argentina from where they get their name. The CT scans also showed the presence of a cocaine metabolite.

The Appeal Committee partially upheld Mr Guerrero's appeal and decided that while Mr Guerrero had some degree of fault or negligence in ingesting the tea, this degree of fault or negligence was not significant. The Appeal Committee reduced the ban from one year to six months. This decision was appealed to the Court of Arbitration for Sport ("CAS") both by Guerrero and by WADA.

Guerrero sought to set aside the decision of the FIFA Appeal Committee and argued that no ban should be imposed. WADA on the other hand sought an increase in the ban to from one year to 22 months. Following a hearing on 3 May 2018 the CAS issued the operative part of its decision on 14 May. The difficulty for Guerrero was that Article 10.5.2 of the World Anti-Doping Code (the "Code") only allowed for the reduction of the ban to a minimum of one year. The CAS partially upheld the appeal filed by WADA in holding that the ban imposed by the FIFA Appeal Committee was insufficient and that a 14 month ban should apply.

There has been wide-ranging support of Guerrero, with the three teams Peru will play in the group stages of the World Cup asking that he be allowed to play. The only remaining route of legal challenge was to the Swiss Federal Tribunal on a point of Swiss law. At 34, this was Guerrero's last chance to play in a World Cup and he duly challenged the CAS decision before the Swiss Federal Tribunal. On 31 May 2018, the Tribunal put a stay on his ban pending the hearing of the full challenge, thus allowing him to play at the World Cup. Critically, this stay was not opposed by FIFA, WADA or the CAS.

It is clear from the Swiss Federal Tribunal decision that the balance of convenience usually lies in favour of letting the athlete compete pending the outcome of the final hearing. If the ban were ultimately reduced by the Swiss Federal Tribunal, he may never get the chance to compete at another World Cup, whereas the remainder of any ban can be served after the World Cup and the final Swiss Federal Tribunal decision.

This very interesting decision left Guerrero free to captain Peru at this summer's World Cup. There was some commentary prior to the competition on the appropriateness of allowing an athlete who was banned by three different hearings panels to continue to play pending the outcome of a further appeal. However, the vast majority of the commentary has focused on the fact that the violation related to a recreational drug, was clearly unintentional and had nothing to do with enhancing performance.