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For Liverpool , the timing could not be much worse.

News of Mamadou Sakho's failed UEFA drugs test has cast a long and lengthy shadow over the club. Today's Premier League clash with Newcastle pales into insignificance as the Reds ponder what could follow.

Sakho has until Tuesday to respond to UEFA's charge, and is almost certain to request that his B-sample is tested. In the meantime, it has been agreed between player and club that he will not be available for selection.

It is believed that the failed test relates to a “fat-burning” substance, and that Sakho believes he has acted in good faith. Nonetheless, the Frenchman faces the prospect of both a hefty fine and, potentially, a long ban too.

What is a 'fat burner'?

That is the question on everyone's lips at present. Of course the exact details are unclear at the moment, but the understanding is that the failed test relates to a “fat burning” substance, with the suggestion being that Sakho was unaware it was on UEFA's 'banned list.'

Fat burners, essentially, are supplements designed specifically to help boost weight loss.

They can boost energy, curb appetite, promote fat to be used for energy, and even increase a person's metabolism and core temperature so that they burn more calories throughout the day.

These kind of products are readily available from pharmacies and specialist health sites.

Is that what Kolo Toure was banned for?

Kind of. Then at Manchester City, Toure failed a drugs test in 2011 after taking water tablets belonging to his wife in an effort to lose weight.

He admitted the violation, and was given a six-month suspension by an independent FA panel, who took into consideration mitigating factors behind the use of the tablets. Had they not, Toure could have faced a two-year ban.

WATCH: James Pearce and Andy Kelly discuss the Sakho news

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Here's what QC Christopher Quinlan, who headed the panel, said at the time: "The criterion in assessing any reduction in what would otherwise be the minimum penalty of two years' suspension is the player's degree of fault. The player accepted he was at fault and with that concession we agree.

"He was at fault in the limited and perfunctory efforts he made in relation to the water tablets; the checks he made in relation to those tablets were inadequate and fell some way below what it would be reasonable to expect of a professional footballer in these circumstances.”

How often are footballers tested?

According to UEFA's official website, Football carries more anti-doping tests than any other sport. In 2014 there were more tests – 31,242 – in football than in any other sport (WADA anti-doping testing figures 2014).

UEFA itself carried out a total of 2,318 tests. This is more than any other regional sports testing organisation. UEFA's testing is not done in isolation. Instead, UEFA works with national anti-doping agencies in European countries to share information and ensure intelligent “targeted-testing” - which was part of Toure's punishment following his failed test in 2011.

Sakho's issue relates to a post-game test following Liverpool's Europa League tie at Manchester United on March 17.

Last season in the UEFA Europa League, controls were carried out at 147 matches, with a total of 588 urine and 31 blood samples collected.

Blood testing is a key part of UEFA's anti-doping programme. UEFA has been testing blood since 2008, starting with the EURO final tournament hosted by Austria and Switzerland. In the 2014/2015 season, UEFA carried out 2,388 tests – 2,073 urine tests and 315 blood tests.

However, it is noted that anti-doping scientists have confirmed that most banned substances are still detected in urine, not blood. Direct blood testing allows detection of some additional substances, but today it cannot simply replace urine testing.

Are Liverpool charged as well as Sakho?

According to UEFA's website: "The provisions included in UEFA's Anti-Doping regulations for teams in the event of anti-doping rule violations are fully compliant with Article 11 of the WADA Code , which states: "Where more than one team member in a team sport has been notified of a possible anti-doping rule violation the team shall be subject to target testing for the event.

"If more than two team members in a team sport are found to have committed an anti-doping rule violation during the event, the team may be subject to disqualification or other disciplinary action."

What happens now then?

The million-dollar question. Sakho has until Tuesday to respond to the charge, and is almost certain to request that his 'B' sample (players always submit two samples when tested) is checked alongside the original sample.

According to UEFA's website: “UEFA adheres to penalties for doping offences as outlined in the World Anti-Doping Code. This includes the imposition of a four-year suspension for a first serious doping offence.

“Earlier this season the UEFA Control, Ethics and Disciplinary Body suspended Dinamo Zagreb's Arijan Ademi for four years following a positive doping test for the prohibited anabolic steroid stanozolol at the Champions League match with Arsenal.”

Sakho's case will be dealt with by UEFA's Control, Ethics and Disciplinary Body, which consists of a chairman, Dr Thomas Partl of Austria, and nine members.

The Body deals with disciplinary cases, both on and off the field, which arise from the UEFA Statutes, regulations and decisions of UEFA that do not fall within another committee or body's competence. The Control, Ethics and Disciplinary Body decides on the halting of proceedings; acquittals; convictions; and the dismissal or acceptance of protests. It also rules on eligibility to play and the admission of clubs to UEFA competitions.

The Control, Ethics and Disciplinary Body shall, as a rule, reach decisions in the presence of all its members, but is entitled to take a decision if at least three of its members are present.

If Sakho's B-sample tests positive, or he admits the charge, then the likelihood is that the defender will face both a large fine and a lengthy playing ban, which would rule him out of the rest of Liverpool's season as well as this summer's European Championships.