The US PGA Tour hit Scott Stallings with a three-month doping suspension Tuesday after he told officials he had inadvertently taken a supplement that was on the banned list.
The American became only the third player punished since the tour adopted the anti-doping policy in 2008 and the 30-year-old is the first tournament winner to receive a suspension.
"I regret my inadvertent mistake in not doing my homework to know for sure what was on the list of permitted and non-permitted substances," Stallings said in a statement.
"I take responsibility and accept the penalty imposed by the PGA Tour."
Neither the PGA Tour nor Stallings revealed what the banned substance was. Tour policy only requires the tour confirm a ban and its length.
"Mr. Stallings self-reported this information after discovering that a supplement that he was taking was on the prohibited list," the tour said in a statement.
"He will be eligible to return on October 7, 2015. The tour will have no further comment on the suspension at this time."
Stallings, ranked 154th in the world, has missed the cut in seven of 12 events and finished outside the top 60 in four others since losing in a playoff at Torrey Pines in February while defending his most recent career title.
Stallings also won the 2011 Greenbrier Classic as a PGA rookie and took the 2012 True South Classic. His best showing in nine major starts was a share of 27th at the 2012 Masters but he will miss all four majors this year.
While Stallings said he immediately reported the violation when he learned of it in February, the PGA did not impose the punishment until July. It offered no explanation for the delay.
"I discovered in February 2015 that I had inadvertently taken a supplement for the prior two months that was not permitted by the PGA Tour," Stallings said in his statement.
"I did so on the recommendation of my physician due to chronic fatigue I had felt over a period of time, not in any way for performance enhancement.
"I immediately self-reported this fact to the PGA Tour, consistent with my values and with the long tradition of self-reporting all rules violations on or off the golf course."
Stallings played no more than five events during the time frame in which he said he was on the banned substance, sharing 14th at the Tournament of Champions and 64th at the Humana Challenge in January.
He missed the cut at Phoenix and was 66th at Riviera around his Torrey Pines title defense.
Stallings won $579,927 from six events starting in February at Torrey Pines, at least some of it after having told the PGA of his violation.