Friday 26 January 2018 19:48, UK
Alexis Sanchez reportedly missed a drugs test on Monday, the day he finalised his move from Arsenal to Manchester United.
Sky Sports News chief reporter Bryan Swanson looks at the Football Association's rules.
How has this happened?
The FA has not confirmed whether Sanchez missed the test at Arsenal's training ground.
Arsene Wenger said it was a "special event" for him to "miss a drugs test" because he was "somewhere else."
The Arsenal boss said the club had not been contacted by the FA or UK Anti-Doping. The FA operates a 'three strikes' policy on missed tests and never comments on alleged anti-doping violations unless a player is punished.
Do players know the rules?
Before the start of each season, the FA reminds clubs and players of its anti-doping 'Whereabouts' programme.
Officials visit clubs and share educational resources with players from different squads, coaches and non-playing staff on a number of subjects, including anti-doping and integrity.
Clubs may also run their own educational courses and sessions.
What if players do not attend?
They should know the rules. The FA even has a 'Whereabouts' video on its website, which clearly reminds them of their responsibilities.
Are clubs to blame if a player misses a test?
Clubs must provide the FA with details of training sessions and home addresses for all squads.
But a player is responsible for notifying the FA if they are unable to follow the squad's training schedule.
How do players inform the FA?
They are required to send a text message or email and provide a one-hour time slot and address for available testing.
They must be available for tests between 6am and 11pm, two hours after they inform the FA of their whereabouts.
What if they keep missing tests?
If a player fails to inform the FA of their absence, or are not present at the time and location provided, they will likely receive a 'Missed Test' strike.
Three strikes within a 12-month period and they will almost certainly face a minimum one-year ban.
