MANCHESTER -- The use of unregulated dietary supplements that may cause players to fail doping tests is a growing problem in professional football, according to a panel of experts and officials at Manchester's Soccerex conference.
A number of players have tested positive for banned substances that were later found to have derived from supplements taken without their clubs' knowledge.
"Supplements are right now a big problem," Emilio Garcia, managing director of integrity for UEFA, said. "The clubs are very well aware of the problem and they need to be extremely careful.
"The cases are difficult because most likely it was a mistake by a player, and from a human perspective it is difficult to deal with these cases."
Liverpool defender Mamadou Sakho, now with Crystal Palace, missed the 2016 Europa League final and the chance to play for France at Euro 2016 after testing positive for a "fat-burner." Sakho was later cleared of any offence after the substance was eventually found to not be on the banned substance list.
Dinamo Zagreb midfielder Arijan Ademi was banned for four years for failing a doping test for a substance contained in a supplement following the Champions League win over Arsenal in 2015, before having his ban halved in March this year.
Football, like many sports, hands down a "strict liability" sentence where ignorance is not regarded as a valid defence for failing a dope test.
"As a club we educate the players and tell them to avoid supplements," Manchester City club doctor Matthew Brown said. "It's for the best of their careers that they avoid supplements, but the problem is policing that.
"We just try to get players to stay off them. The problem is that we have had quite high-profile international players that might go on loan or abroad. At City we have clubs internationally, and we might have players at New York City using a supplement and they are told it's fine in the United States, and in the UK it may not be fine. So I get them to run everything, whether it's medication or supplements by myself or a nutritionist within the club."
Michele Veroken, a sports ethics and anti-doping adviser who runs Sporting Integrity, said: "I've seen too many footballers fall foul of these doping regulations and once it's there, it takes the power of legal argument to reduce the sanction if not excuse the offence."
Veroken, an ex-head of doping at UK Sport, added: "The difficulty in football and sport in general is that supplements companies are very keen to offer sponsorship deals.
"Some have reputable backgrounds, and the question is whether the footballers actually need this. Nutrition has changed around football clubs but unfortunately, the mystique about supplementing your diet and making yourself better still exists."
Garcia declared that UEFA was satisfied that doping was not too much of a problem in football.
He said: "Looking at the figures published by the World Anti Doping Agency and UEFA in our competitions, I would say the problem is limited and most of these cases are linked with recreational and social drugs.
"We conduct 3,000 doping tests, and one third are out of competition; we have a strong anti-doping programme, we need to fight against this cancer. Football is 11 players, a team sport, and that helps a lot in this fight. Doping is in principle a limited sport in terms of doping."