Liverpool defender Mamadou Sakho has "let himself and his club down badly" with his failed drugs test, according to former Reds centre-back Jamie Carragher.
UEFA is currently investigating a possible anti-doping violation by Sakho, who failed a drugs test after Liverpool's Europa League second-leg tie at Manchester United last month.
It is understood the failed test is associated with a fat-burning supplement and Sakho, who was pulled out of Saturday's game against Newcastle but has not been suspended either by Liverpool or UEFA, now faces an anxious wait to see whether he will receive a playing ban.
The France international is likely to ask for his B sample to be tested, with the deadline for that request set for Tuesday.
Carragher, whose one-club playing career came to an end with his retirement in 2013, told Sky Sports on Monday: "It's a nightmare for the player.
"I think he's let himself and his club down badly, because he was becoming a really important player for Liverpool.
"There are serious questions to be asked of him.
"He's had a great campaign. He's one of the leaders of the team now. You think of the Borussia Dortmund game, the Manchester United games especially, how well he played in those. It's now how Liverpool fill that void.
"But for a professional footballer -- or any sportsman, really -- it's not acceptable."
The British Dietetic Association (BDA) says the Sakho case should warn players there is no "magic answer" to solving weight issues.
"The culture has moved away from food and towards supplements and using potions and lotions," BDA spokesperson Aisling Pigott told Press Association Sport.
"But athletes have to be extremely careful because unless those supplements are legal there is a big risk that they could be contaminated.
"I see a lot of under-18 footballers today talking about supplements, but you can not afford to go down the illegal route.
"Fat-burning supplements are difficult to monitor and if there was anything reliable to burn fat, it would be introduced across the board in the health service because of the [obesity] crisis we have.
"People are looking for the magic answer all the time, but it does not exist."
Sources say that Sakho might have failed a drugs test as some fat-burners consist of mild-to-moderate stimulant-type drugs such as ephedrine.
Those drugs could enhance performance by increasing metabolism and providing extra energy, but the BDA say illegal supplements can also have the opposite effect.
"You can have a bunch of placebo ingredients cooked up, but these are generally high in caffeine and can have a negative effect on performance," said Pigott.
"They can affect sleep and make an individual anxious.
"They are not legal and safe, and I would be surprised to hear of any elite athlete taking these supplements.
"It's also vital that any organisation knows the risk of supplements, and football clubs must spend more time on education and have a broader level of knowledge.
"They must make it clear that it's not a good idea for a player to go into a shop and buy a fat-burning supplement, because it just doesn't work."