The International Equestrian Federation (FEI) suspended the 33-year-old former European champion, making him ineligible to compete for Germany in the individual Olympic final in Hong Kong on Thursday, Aug. 21.
The banned substance capsaicin was found in Ahlmann's horse, Coester, on Sunday, a day before the team show jumping finals in which Germany finished fifth.
Tested positive for chilies?
Under new rules, riders and their horses can be suspended if their A samples are positive. The result of Coester's B sample is due to be announced by the Hong Kong Analytical Laboratory on Friday.
All German horses were tested for drugs before they left for Hong Kong at the beginning of August and nothing untoward was found, said the team's veterinarian, Bjoern Nolting.
Nolting said he was concerned that the positive results may have been a result of chilies applied to the skin as a superficial treatment.
"It has a primarily blood circulation-promoting effect," said Nolting, noting that capsaicin is an extract derived of chilies.
Other horses fail doping test
Germany's equestrian team is not the only one to be experiencing this problem. Norway could lose the bronze medal it won in the team show-jumping event after rider Tony Andre Hansen's horse Camiro tested positive for the same banned substance capsaicin, the International Equestrian Federation (FEI) confirmed Thursday.
If the B sample confirms the failed dope test then Hansen's team drop off the medal podium and will be replaced by Switzerland. Gold in the event Monday went to the United States while Canada took silver.
Lantinus, the horse of Ireland's Denis Lynch, and Brazilian Bernhard Alves' horse Chupa Chup also failed dope tests, making Lynch ineligible for Thursday's individual competition.
It's not the first time that the Olympic equestrian events have been marred by doping. At the Athens Games four years ago, Germany was forced to hand back its team gold after failing to register medication given to one of the horses.