Reports in France claimed Sakho tested positive for a fat burner after the Reds' 1-1 draw against Manchester United in the Europa League on March 17, but Liverpool have not confirmed those suggestions.
"Yesterday, Friday 22nd April 2016, a formal communication was received from UEFA stating that they are investigating a possible anti-doping rule violation by Mamadou Sakho," a statement on Liverpool's website read.
"The player will respond to UEFA on the matter. The player is currently not subject to any playing suspension”, the statement continued."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."
Sakho, who has 28 caps for France, is said to be ready to request his B sample be tested, with the deadline for that request set for Tuesday. Some watchers believed the 26-year-old centre-back did not know the composition of the weight-loss supplement and he was said to have informed the club that he acted in good faith.
Sakho's Liverpool teammate Kolo Toure was suspended for six months after failing a drugs test while playing for Manchester City in February 2011. The Cote d'Ivoire defender had tested positive for bendroflumethiazide, a weight-loss drug contained within "water tablets" and apparently recommended by his wife.
If Sakho is unable to prove his innocence it would be a significant blow to Liverpool manager Jurgen Klopp as the former Paris Saint-Germain player has formed a solid partnership alongside Dejan Lovren in recent weeks.
Sakho, who moved to Anfield for 22 million euros in 2013, is in danger of missing the finale to Liverpool's Europa League challenge, with the team facing Villarreal in the semi-final first leg in Spain on Thursday. He could also miss the Euro 2016 finals in France if he were found guilty and given a long ban.
Sakho started France's last match, a 4-2 friendly win against Russia at the end of March.
Liverpool have made a swift response to the alleged doping violation and say they have fully consulted with UEFA and the English Football Association. While Sakho is technically still available to play, it was felt by Klopp and Liverpool's US-based owners that the best course of action was to withdraw the player from selection and it is understood Sakho agreed with the decision.
Liverpool won't face disciplinary action as that can only happen if two players in the same season are found to have had doping violations.