Madrid - Reigning champion Chris Horner was pulled out of the Tour of Spain on Friday as treatment he was receiving for bronchitis was deemed a health risk by his Lampre-Merida team.
The American was given doses of cortisone after suffering bronchitis during the Tour de France and Tour of Utah and had been cleared to race by the International Cycling Union (UCI).
However, the team have adhered to the regulations of the Movement for Credible Cycling (MPCC) after Horner showed a cortisol level lower than the minimum recommended amount.
"Of course I'm sad about this news. Trying to defend the 2013 title Vuelta was my main target in the season, the team signed me with the aim of being competitive in the Spanish race, but I accept the decision linked to the MPCC's rules," Horner said in a team statement.
"This bad bronchitis has caused me a lot of problems. I've been suffering for it for weeks and this treatment could have allowed me to solve the problem.
"UCI gave authorisation for the treatment, I could race according UCI rules, but my team is member of MPCC, I understand it and we all must accept this situation without regrets."
Horner produced a major shock when he became the oldest ever Grand Tour winner at 41 last year by beating Tour de France winner Vincenzo Nibali to victory in the gruelling three-week race.
This year's event gets underway on Saturday in Jerez with last year's Tour de France winner Chris Froome and Giro d'Italia champion Nairo Quintana the favourites for victory.