Shoemark has not ridden since November 24, with the British Horseracing Authority confirming the rider has been stood down for the duration of its investigation.
The jockey, whose main backer is Roger Charlton, was informed of his test result at Kempton on November 28 and "accepts full responsibility for his mistakes", according to Professional Jockeys Association chief executive Paul Struthers.
In a statement, Struthers said: "Through the PJA, Kieran would like to sincerely apologise for his actions that led to the positive sample. In particular, we would like to apologise to Mr Charlton, the owners and team at Beckhampton Stables and his family.
"He accepts full responsibility for his mistakes and would also like to apologise to his colleagues and the wider sport.
"He is determined to face-up to any issues he may have and with the support of his family and the PJA is already fully engaged with the support structures the PJA has in place.
"The PJA completely supports a robust anti-doping protocol for its members and the BHA has our full backing in enhancing the testing that already takes place.
"But like other player associations, we believe it is important to have in place the necessary support structures for those members who want and need help. We can condemn the behavior without condemning the individual.
"We would request that his privacy and that of his family is respected whilst he is accessing the support he needs."
The BHA would not comment on the specifics of the case, but said: "Kieran Shoemark has been stood down from riding for an unspecified, interim period while an investigation takes place.
"The BHA cannot comment further on the details of investigations."
Shoemark, 22, rode 65 winners last year - including one at Royal Ascot for Charlton - and came close to landing the champion apprentice title, missing out by just one win to David Egan.
However, after making a decent start to 2018, he suffered a fall at Lingfield in June which put him out of action for several weeks with broken ribs and a punctured lung.
Shoemark eventually returned to the saddle in late August, with the Charlton-trained Projection giving him a Group Three win at Ascot in October.
