Santana, 40, issued the statement saying he had checked into the program Saturday.
"I do not know how long this stage of my journey will be. I apologize to my family, friends, the mayor, city council, CAO staff and the entire city family for my irresponsible behavior," Santana said in a statement.
Santana said his top assistant Ray Ciranna will be acting city administrative officer.
Santana has been on the job since 2009. He's been addressing the city's $212-million budget gap, which is expected to more than double in the next budget year.
