Liverpool boss Brendan Rodgers will speak to Raheem Sterling after the forward was pictured apparently taking nitrous oxide.
Images published by The Sun appear to show the 20-year-old England forward taking the legal substance, commonly known as laughing gas.
Sterling scored in Liverpool's 2-0 win over Newcastle at Anfield on Monday.
"I don't think it is something you should be doing, it's as simple as that," said Rodgers.
The incident comes after Sterling admitted rejecting a new £100,000-a-week Liverpool contract in an interview with the BBC.
"We want players here who are super-professional and focused on their football - I know he is," added Rodgers.
"It's not just on the field, it's off the field as well but I will speak with him on it and we'll move on.
"Young players make mistakes. As long as they learn from them, that is what is important."
Nitrous oxide is often used in surgery and dentistry for its anaesthetic effects but it is also a popular legal high as it can make people feel relaxed, euphoric and giggly.
A Home Office campaign last year on the risks of legal highs showed it was the second most popular drug among young adults after cannabis.
The images are the second time in as many days that Sterling has featured in the news pages. The Sunday Mirror published pictures of the player allegedly smoking a shisha-pipe over the weekend.
Speaking on Sky Sports, former Liverpool defender Jamie Carragher admitted he made mistakes as a young player, but added: "Raheem needs to be on the back pages, not the front pages."
|Former West Indies cricket captain Sir Viv Richards|
|"When you are thrust into the limelight as quickly as some of these guys are it is important to keep a level head and to be worried about your surroundings. Your friends are always of vital importance, because sometimes they can guide you in the wrong direction. But if everything around your friends is positive it's of enormous help."|