Olympic 400 metres champ LaShawn Merritt of the United States has been suspended after testing positive for an anabolic steroid contained in an over-the-counter penis enlargement product.

Merritt, who is also the world champion at 400 metres, said in a statement via his lawyer that he was "deeply sorry" at failing three doping controls for the banned substance dehydroepiandrosterone.

Olympic champion LaShawn Merritt. Credit:AP

The 23-year-old American, who faces a two-year ban, said: "To know that I've tested positive as a result of a product that I used for personal reasons is extremely difficult to wrap my hands around.

"I hope my sponsors, family, friends and the sport itself will forgive me for making such a foolish, immature and egotistical mistake. Any penalty that I may receive for my action will not overshadow the embarrassment and humiliation that I feel inside.