The league did not reveal what substance was discovered in the player's system. Konopka, a 33-year-old native of Niagara-On-The-Lake, Ont., later issued a statement through the NHL Players' Association.
"I deeply regret that I have tested positive for a prohibited substance in violation of the NHL/NHLPA performance-enhancing substances program," he said. "I accept full responsibility for this error. As a professional athlete I am responsible for what I put in my body, and I am to blame for this mistake. I want to make it clear that this violation occurred because I ingested a product that can be purchased over-the-counter and which, unknown to me, contained a substance that violated the program. Unfortunately, I did not take the necessary care to ensure that the product did not contain a prohibited substance.
"I want to stress, however, that I did not take this substance for the purpose of enhancing my athletic performance. I apologize to my teammates and to the Buffalo Sabres organization, as well as to my friends and family. I have learned a great deal from this experience, something I hope to be able to share with younger players. I will accept this suspension and continue to work hard to prepare for the 2014-15 season."
The NHL said the 20-game ban will be enforced once the player signs a new contract. The Sabres, who did not make the playoffs, claimed the journeyman off waivers in January after he was released by Minnesota.
Konopka had three points and 88 penalty minutes in 59 games this past season. His future was already uncertain in Buffalo because he is eligible to become an unrestricted free agent in July.
Under the terms of the collective bargaining agreement, the suspension is accompanied by mandatory referral to the NHL/NHLPA program for substance abuse and behavioural health for evaluation and possible treatment.
In 346 career NHL regular-season games, Konopka has 30 points (12 goals, 18 assists) and 1,082 penalty minutes. He has also played for the Anaheim Ducks, Columbus Blue Jackets, Tampa Bay Lightning, New York Islanders and Ottawa Senators over his nine-year career.
Konopka has two assists in eight career playoff games.
With files from The Associated Press.