The alleged offence relates to the Reds' Europa League last-16 second-leg tie at Manchester United and is believed to concern a so-called 'fat-burning' substance.
Press Association Sport understands the France international is highly likely to request his B sample be tested, with the deadline for that request set for Tuesday.
Sakho has not been suspended either by the club or UEFA, but it is understood that after discussion between the club's owners Fenway Sports Group, manager Jurgen Klopp and the player himself it was decided the centre-back should not play in the immediate future, even though he is still eligible for Premier League matches and Thursday's Europa League semi-final first leg in Spain against Villarreal.
He was therefore not considered for selection in Saturday's game against Newcastle.
Liverpool issued a brief statement confirming an apparent doping violation.
"(On Friday) a formal communication was received from UEFA stating that they are investigating a possible anti-doping rule violation by Mamadou Sakho," it said.
"The player will respond to UEFA on the matter. The player is currently not subject to any playing suspension.
"However, the club, in consultation with the player, has decided that while this process is followed the player will not be available for selection for matches.
"There will be no further comment at this time."