Prosecutors have dropped a doping investigation of a German doctor who injected patients with their own blood after giving it ultraviolet light treatment.

Among the patients were 30 athletes.

The Erfurt prosecutor's office said Wednesday there was no evidence of any wrongdoing and that a "conviction was not to be expected."

The prosecutors said they had no evidence the athletes used the method to enhance their performances, something they have denied.

Doctor Andreas Franke has used the procedure for years and made no secret of it at the Erfurt Olympic training center where he works.