The 42-year-old American was pulled out of the race which starts on Saturday by Team Lampre-Merida as although his cortisol level is not illegal it is below that specified by the Movement for Credible Cycling (MPCC).
"It has been necessary for a change of programme," Lampre-Merida said in a statement.
"Even though Horner would be allowed to start the race as far as the UCI is concerned, the team are obliged to respect the rules and regulations of the MPCC organisation in a completely voluntary decision."
Horner used oral cortisone to treat lingering bronchitis after the Tour de France which has caused the condition and he had been given therapeutic exemption by the UCI.
The American made history last year when his victory in the Vuelta made him the oldest grand tour winner aged 41 but a repeat this time around was unlikely due to the strong field led by Chris Froome and Nairo Quintana.
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