Gallagher returned a positive A-sample result for the blood booster EPO (Erythropoietin) in an out of competition test at a July training camp in Italy.
“Although the B sample is yet to be tested, It will not change the situation. I have done the wrong thing. People deserve an explanation and I want to give it,” he said in a Facebook post.
“Huge apologies to all the people I have raced during this period, everyone is entitled to an honest performance and I know how cheated people would feel.
“Sorry to the Paracycling team who are about to go into the biggest event with this blow and an unfair link to my stupid actions.
“The support staff, coaches and athletes were all so supportive of me and it breaks my heart to have let them down. I will be following their results and wishing them all the best I can. The apology also extends to the whole Paralympic team.”
Gallagher, who won gold medals in London 2012 Olympic Games and earlier in Beijing, said a range of personal issues led him to the drug in the lead up to Rio.
“Although there are never any excuses for something like this, you do self-analyse and look for reasons. No one in my life saw this coming and if you asked me over a year ago I would never have seen it coming either.
“Worsening mental health issues (depression) and other personal issues in life led to an inability to train and hurt myself like I used to. With the expectations of living up to past performances in a Paralympic year and the need to do so to pay my bills, the pressure mounted.
“Rather than seek help l self-medicated to motivate, crossed the dark line, took short cuts and cheated. I had many ways of justifying this use for Para-sport which in hindsight were merely just dark, paranoid and selfish justifications to talk myself into it.
“I think complete honesty is my first road to recovery. I cheated at sport and put an end to a chapter in my life which has given me so much by my own wrongdoing. I will need to accept and live with that.
“But all this doesn’t sadden me as much as the feeling of letting the important people in my life down.”