GB cross country international Luke Traynor says he made "an incredibly stupid and uncharacteristic mistake" after testing positive for cocaine.
The 26-year-old Scot has been provisionally suspended from competition by UK Anti-Doping and could be banned for up to four years.
Traynor issued a statement on Twitter, stressing the drug was taken socially and not to enhance performance.
"This was devastating news for me and I take full responsibility," he said.
Traynor, who has the opportunity to respond to the charge of violating anti-doping rules, competed for Britain at the World Cross Country Championships in March, finishing 109th, and was 38th at the World Half Marathon Championships last year.
He revealed he had been notified that he had tested positive "for benzoylecgonine, a metabolite of cocaine".
Traynor added: "I am sorry to my family, friends, coaches sponsors, and anybody who has supported my progress at any stage of my career.
"I want to make it completely clear that my violation was in no relation to sport or enhancing performance.
"This happened as a one-off and in a purely social situation with a drug I should never have taken."
Former Hibernian striker Jamie Insall returned to football this year with Welsh Premier League club Connah's Quay Nomads following a two-year ban for a similar incident.
Traynor, who runs for Giffnock North and Tulsa University, said he has "accepted my mistake" and needs to come to terms with "the final punishment".
"I have cooperated fully with all relevant bodies and will now face the severe consequences, the extent to which is still not certain but could be up to a four-year ban," he said.
"I live a sporting lifestyle to compete. Athletics is my passion. It's all I think about, it's what gets me out of bed in the morning.
"I am yet to come to terms with the fact that I have ruined this for myself with one senseless act.
"I only ask that people understand that this was a stupid mistake, had nothing to do with trying to enhance performance and, in fact, had the opposite effect."