Ghana midfielder Michael Essien has missed Real Madrid's recent matches after having an allergic reaction to medicine he was given by the club.
Essien has not played for the Spanish champions since 11 November after playing in makeshift roles for the club.
Last Tuesday, coach Jose Mourinho disclosed the 29-year-old had fitness problems after he was excluded from the squad which played Manchester City in the Champions League.
But a press conference on Friday, assistant coach Aitor Karanka explained the situation further.
"He has had an allergic reaction to some medicine," Karanka said.
Essien is on a season-long loan move from English side Chelsea and has been impressive so far.