Tom Bassindale, a forensic toxicologist and doping expert at Sheffield Hallam University, says he has mixed feelings about Powell's return to the track.
"I'm a bit torn," he said. "If people have been taking something like EPO, my heart does sink to see them back in competition because they may still be benefiting from the effects.
"But a mild stimulant like oxilofrine may be less effective than caffeine. I don't like to see people breaking the anti-doping rules, but clearly some substances are more effective than others."