Sevilla midfielder Samir Nasri is being investigated by the Spanish Anti-Doping Agency (AEPSAD) over treatment he received at a Los Angeles clinic.
The Drip Doctors clinic posted a tweet saying the on-loan Manchester City star had been given an immunity IV drip.
It triggered a series of embarrassing tweets, which were subsequently deleted, leading to the Frenchman claiming his account had been hacked.
The drip given to Nasri, 29, may break World Anti-Doping Agency (Wada) rules.
The immunity IV drip involves one litre of hydration and is designed to “combat superbugs and common viruses”, according to the Drip Doctors website.
Wada place a 50ml limit on IV infusions for active athletes in six-hour periods unless it was “received in the course of hospital admissions, surgical procedures or clinical investigations”.
A statement from AEPSAD said it had “commenced an investigation into treatment used by football player Samir Nasri”.
It added: “AEPSAD wants to gather all the information and check what happened. We must be very prudent at the moment. We can’t prejudge the player.”
Former Arsenal player Nasri, who has 41 France caps, joined Manchester City in 2011 but left in August for a season-long loan with the Spanish side, who are third in La Liga.