Sevilla midfielder Samir Nasri has been asked to explain his use of intravenous drip treatment received at a Los Angeles clinic.
The 29-year-old, who is on loan at the Spanish club from Manchester City for the rest of the season, made headlines this week when caught in a Twitter row over an alleged tryst.
A series of tweets claimed that Nasri romped with a medic who gave him an IV drip as part of a "full sexual service", but the Frenchman later denied the allegations and insisted that his account had been hacked.
However, Nasri may still have violated anti-doping rules and is now at the centre of a probe launched by the Spanish Agency for Health Protection in Sport (AEPSAD).
AEPSAD director Enrique Gomez Bastida said: "It is necessary to know in what frame and in what situations [the IV drip] occurred. We will verify the facts to know where we are.
"You have to be careful with the rumours. In the Nasri case there is no sanctioning procedure at the moment, so there could be no interim injunction."
Nasri, who has reportedly been valued at £21m by parent club City, is free to continue playing while the investigation is ongoing.
Daniel Lewis
