Hill's ban began Friday night when the Islanders faced the Buffalo Sabres in Game 5 of the first-round Eastern Conference playoff series. The earliest Hill could return to the lineup would be the Stanley Cup final.
He is the first player to be suspended under this program.
Islanders spokesman Chris Botta said Hill didn't travel with the team for Friday night's game. He added he didn't know whether Hill had returned to Long Island.
The nature of Hill's infraction was not immediately clear.
"It's unfortunate it happened, and we certainly hope that we won't have many more violations of our policy as we move forward with the program," NHL deputy commissioner Bill Daly told The Canadian Press. "But this result shows that our program is working as it was designed and intended to work."
Islanders general manager Garth Snow said the team supported the NHL's decision to suspend Hill.
"The New York Islanders do not support the use of performance-enhancing agents in any form," Snow added. "As for tonight, we have a game an hour away, and that's what we're going to be focused on from here on out."
Snow spoke for 30 seconds, reading from a brief statement, and did not take any questions.
Islanders coach Ted Nolan declined comment outside the club's locker room. Team spokesman Chris Botta said Hill travelled home to Long Island before the game.
New York started the day in a 3-1 hole to top-seeded Buffalo in the best-of-seven matchup.
Hill, 37, showed no sign Thursday, before the Islanders flew to Buffalo, that anything was amiss. He joined the rest of his teammates in talking about putting several controversial calls in consecutive home losses this week behind them as they prepared for Game 5.
"If we can use it as a motivational tool, that would be great. If we can't, we just have to put it behind us," he said Thursday. "It doesn't matter what happened in the last game, it doesn't matter what happened in the previous games.
"We have to focus all our energy on that one game. If we do that we'll have a good outcome."
Hill had one goal and 24 assists in 81 games this season, his first with the Islanders.
He did not record a point in the Islanders' first four playoff games against Buffalo.
As part of the new collective bargaining agreement that ended the year-long lockout in 2005, a player receives a 20-game suspension for a first positive test and is subject to a mandatory referral to the league's substance abuse-behavioural health program for evaluation, education and possible treatment.
Every NHL player can be given up to two "no-notice" tests every year, with at least one conducted on a team-wide basis. Players can be given a "no-notice" test at any time.
Sabres defenceman Brian Campbell said Buffalo players were tested twice during the regular season.
"Who knows what happened?" Campbell said, before the game. "I feel bad for him that it's happened.
"You don't know what it is until everything comes out. ... You just have to be careful all the time."
The NHL's anti-doping policy has been called into question by Montreal lawyer Dick Pound, chairman of the World Anti-Doping Agency.
He said last year the policy was "very seriously flawed" and made headlines in November 2005 when he estimated that one third of NHL players were likely taking performance enhancing substances - mainly stimulants.
The NHL does not test for the drugs on WADA's list of banned substances that are prohibited only during competition, such as stimulants. Some cold remedies that contain stimulants, such as ephedrine, are suspected to be widely used by hockey players.
Players as well as league and union officials unanimously denied Pound's claims.
Defenceman Bryan Berard of the Columbus Blue Jackets and Colorado Avalanche goalie Jose Theodore both failed out-of-competition tests administered by their respective national anti-doping organizations in November 2005.
But neither was suspended by the league because the failed tests happened before the NHL established its new policy.
Hill, a native of Duluth, Minn., was drafted by the Montreal Canadiens in the eighth round of the 1988 NHL entry draft. In 841 career regular-season games, Hill has 60 goals and 229 assists for 289 points.
Earlier this year, Islanders' forward Chris Simon received a record ban of at least 25 games that ended his season and could carry into the next campaign, for striking Ryan Hollweg of the New York Rangers with a two-handed swing of his stick.
Simon served the 20th game of the NHL's longest suspension for on-ice violence Friday. He is ineligible to return to the playoffs and will have to sit out any remaining games next year should the Islanders fail to play five more this season.