The ban will prevent her from taking part in competitions either at home in Germany or abroad.
Her horse, El Santo, tested positive for Cimetidine after a disciplinary committee ordered his urine be tested in 2012, the country's equestrian umbrella organisation the FN said on Tuesday.
Cimetidine is generally used to treat stomach pain in horses, but it not allowed to be used by professional riders competing in German competitions. It is not classed as a “doping” drug, but is listed as a banned medicine. It is allowed at a international level.
Its use was, the FN said, a negligent breach of Werth's duty of care. She is not only banned from competing for six months but must also pay a fine of €2,000.
Werth said she had not ordered the medicine be administered but suggested that El Santo had drunk contaminated water in his stall. The FN's disciplinary committee said this was “highly unlikely.”
This is 44-year-old Werth's second sixth-month ban from competing. In 2009 she was found guilty of giving her dressage horse Whisper psychotropic drug Fluphenazine.
READ MORE: Lucky escape for girl and her little pony