The organisation representing professional footballers worldwide called for an urgent inquiry on Thursday into medical treatment administered to players under the care of at least four first-division football clubs in Cyprus.
In a statement, FIFPro said players at these clubs have had unidentified substances administered, some of them intravenously, to aid their physical recovery.
Three of these players have been diagnosed recently with a serious heart condition.
The world player union has written to the Cyprus Football Association, the Cyprus Sports Organization and UEFA asking for their immediate assistance.
Because of concerns there is a link between the unidentified substances and the serious heart condition of the three players, FIFPro is calling for football authorities to help identify what was administered to them and to establish if their teammates are in danger.
The substances are understood to have been administered since the start of the 2017/18 season. Some of the players under the care of the four football clubs last season have left Cyprus to continue their careers elsewhere.
FIFPro and the Cyprus player union (PASP) are providing medical and financial support to the three footballers diagnosed with a serious heart condition.
Two players from first division club Alki, Panayiotis Frangeskou, 27, and Panayiotis Loizides, 23, asked police on November 1 to investigate whether recently discovered identical heart problems that forced them to quit the game were a result of intravenous injections containing unknown substances administered to them by the club officials during the previous season.
Police have since completed their investigation and handed the case file to the attorney-general who will decide what the next steps would be according to the findings, which have not been made public.