Michael Ellerton said steroid hormones prednisone and prednisolone appeared in his system because of mouth ulcer medication he was taking.
UK Anti-Doping (Ukad) refused his retrospective application for a therapeutic use exemption.
But Ukad added it was "satisfied Ellerton did not act intentionally".
Ellerton tested positive in an in-competition test at the Port Talbot Wheelers Cycling Club's annual 25 Mile Time Trial on 11 September 2016.
A ComRes poll for BBC Sport released earlier this week of more than 1,000 members of sports clubs and teams found more than a third (35%) of amateur sports people say they personally know someone who has doped.
A 55-year-old amateur cyclist has been banned from all sport for two years after testing positive for banned substances at a race in September.