San Francisco 49ers long snapper Kyle Nelson has been suspended without pay for 10 games for violation of the league's policy on performance-enhancing substances, the NFL announced.
This is the second offense for the 32-year-old Nelson, who was also banned for violation of the same policy prior to his time with the 49ers.
Nelson will begin serving the suspension immediately.
"I have been made aware by the NFL that I tested positive for a banned substance and as a result will be suspended," Nelson said in a statement. "I have been taking the same supplements for my entire time with the 49ers, while undergoing often simultaneous testing by the NFL with no issues.
"I do not deviate from my nutrition routine, and I would never knowingly take a banned substance. I am currently having the supplements tested, and I hope to uncover the cause of the positive test. When the product is identified, I fully intend to seek legal action against the manufacturer to protect against this happening to other players in the future."
Nelson, who will become an unrestricted free agent after the season, will miss the final four contests of 2018 and the first six of the 2019 campaign.
"We are certainly disappointed to be losing Kyle for 10 games," general manager John Lynch said in a statement. "Hopefully this serves as a reminder to all of our players that they need to take the proper steps to know exactly what they are putting in their bodies."
Nelson has played in 76 straight games with the 49ers since signing with the team in 2014. He played for the Washington Redskins in 2013.